Keepers of the Feathers

The Keepers of the Feather is a secret society comprised of wereravens. Typical Barovians and Vistani are unaware of them. The book makes it quite clear that Strahd is aware of them, but doesn't really care. They would like to keep it that way. As wereravens, they are immune to nonmagical damage and most other types of damage; however, they can be overwhelmed and then killed in other ways. And in Barovia, they are heavily outnumbered by things that would kill them if given the chance. Thus, the Keepers are likely to avoid direct combat whenever they can. If they do fight, are likely to flee if there's a chance they'll be overwhelmed.

Spy Network

Functionally, the Keepers of the Feather serve as a distributed spy network that uses the vast host of ravens already living in the valley as both spies and messengers. Because the wereravens can speak with the ravens, they can even have ravens carrying messages verbally in the "raven language" as long as the message is quite simple.
1. All ravens in Barovia reflexively defer to the Keepers of the Feather, but not all of them are actively 'working for them.'
2. The Keepers deploy large numbers of normal ravens around the valley with varying instructions. They could be assigned to warning travelers of danger, looking for someone or something, watching for specific things to happen, or simply reporting back if something odd happens. However, reports from ravens are simplistic--they're really smart birds, but are still just birds.
3. Around the valley are Nests--Places where a group of wereravens live and 'command' a flock of Ravens. Anything of interest that a raven comes back with is filtered through their local wereraven, who decides if it is actually something significant (a raven may well inform you that it just saw a deer get wrecked by wolves, and it came back to invite you to come get some food).
4. On a small scale, the leader of an individual Nest has a lot of autonomy. If something merits further investigation, he will deploy a wereraven to go check it out in raven form.
5. Anything truly significant will be communicated up the chain of command.
Limited Intel There are limits to what the Keepers can find out with their normal network. Anything happening outside is easy to spy on, anything happening near windows is easy to watch (but hard to hear), and a raven going inside a structure is conspicuous and will probably cause a negative reaction. Thus, the Keepers are aware of any movement between the towns, and generally aware of the goings on in the wilderness and outdoors. However, things that happen indoors are harder for them to know about.

How They Can Help

The wereravens want to see Strahd fall. Any party that clearly seems bent on that will have their support, as long as they don't do something like burn down the winery or the tavern, or kill a bunch of wereravens.
Early on, they will try to help without revealing who or what they are. Members of the Keepers of the Feather may act as 'quest givers' hoping to point the party in the direction of people who need their help, or allies that may aid them, or evils that need a good stabbing. They can provide intel to the party in the guise of rumors they heard (this will be especially believable coming from Danika or Urwin--as Players are used to bartenders being quest dispensaries. And, of course, as it is called out in the Vallaki chapter, if the party is stripped of gear, the Keepers will steal it and return it.
Later on, they may reveal themselves to the party--even if they don't reveal that they are wereravens. They can make themselves out to be a secret society that opposes Strahd, and trains ravens to help them (all technically true). This gives the party access to a spy network spread throughout the valley. Once this has happened, the party can use the Keepers to send messages around the valley, get places scouted out before they go, and be notified if something important is happening somewhere other than where they are.
The wereravens should be hesitant to reveal themselves or to physically commit to fighting alongside the PCs. After all, they are far from the first adventurers the Keepers have encountered and tried to help. Their goal is to operate as an intelligence network and, even if the PCs fail (like so many before them have), they'll be able to continue functioning. In fact, it's likely that the wereravens knowledge of some locations is supported by the fact that adventurers have died in those places.
A final method they may be able to use to help is emergency evacuation. A pair of them, working together, can carry pretty much any PC. If they can lift them, they can fly, and thus potentially snatch imperiled PCs away from harm at the last minute.

Specific Keepers

The only members of the Keepers of the Feather that we know by name are the Martikovs.

Davian Martikov - The Patriarch

Davian is a cantankerous old man who is likely a lot more bark than bite. He's called out as being suspicious, slow to trust, and cranky...but he gives away the vast bulk of the wine that the winery produces. The way I swung this in my game is that taverns get shipments of wine for free...but if a private individual wants to buy wine directly, he charges them for it. Essentially, if you want your own private store of wine, you have to pay for it.
For the sake of the Keepers, this gives him a very practical advantage. The Martikovs of the Wizard of Wines are well beloved and welcome in every town in the valley. Even the insular Krezk welcomes the Martikovs, because they come bearing large quantities of wine. On top of this, it's very common to shoot the breeze with a familiar traveler, especially while you're--say--helping them unload barrels of wine, then sitting down to share a cup or two before they set out again. Through this, Davian and his kids are often kept up to date on the latest gossip and news around town that would be hard to gather from passive observation in raven form.
Davian himself is...a tough man to get along with. He's old, set in his ways, and cranky to the point that Urwin considers dealing with him to be a test of the PCs' competence. So play this up.

Danika Martikov - Barkeep

Danika is probably the most personally informed member of the Keepers of the Feather and should be one the PCs get plenty of interaction with. Why? She's the bartender at the inn in the 'hub' city. Vallaki is the trade hub of the valley and, assuming it doesn't get wrecked by its internal instability, is a likely site for the PCs to use as a base of operations. Thus, when the party wants food or wine or a place to stay...they go to Danika.
It's a very, very common trope that people talk to their bartender about things they won't talk to other people about. Particularly when they get a bit drunk. And in Vallaki, the largest town in the valley, that's Danika. On top of that, her husband is a very high-ranking member of the Keepers and she gets that intel as well. Danika knows her role and plays it well. She's personable, always willing to lend a sympathetic ear, and has a reputation for discretion...because she only shares 'important' info with the Keepers, who are all very close mouthed about what they know. She should be friendly to the party from the moment they arrive, be an enthusiastic audience for them to tell stories to (possibly offering them a glass of wine in exchange for the 'entertainment').
As a result, Danika likely knows more about people's personal lives than they might be comfortable with while sober. She can use this vast array of information--both from being a bartender and being highly placed in a spy network--to act as an informant and quest-giver to the party.

Urwin Martikov - The Cook/Spymaster

Urwin Martikov is not as sociable as his wife (though he's still friendly), which is why he mostly stays in the kitchen and cooks. It helps that he's a pretty good cook, and makes the best of the rather limited fare available in the valley.
Urwin is also one of the highest ranking members of the Keepers of the Feather and is likely the one the party will end up interacting with should the Keepers decide to properly reveal themselves as a spy network. Urwin is well liked in town (he's the tavern's cook, after all) but is likely to avoid landing in any position of power within the town if he can help it.
Some parties I have heard tell of (including mine) view Urwin as being a good candidate for Burgomaster should Vargas and Fiona both end up in the ground...but he's a spymaster. He does not want to be Burgomaster and be saddled with all the day to day work of being in charge of the town, nor does he want the exposure of being someone so important.
In my game I portrayed him as a genial, but not overly loud individual--a man who chooses his words carefully. He is very much in love with his wife and adores his boys...and cheerfully teases all three of them, and they answer in kind.
When he's not cooking, Urwin also handles the inn's deliveries--both receiving goods, and delivering any special orders put out. He has a tendency to get roped into helping with preparing for the Baron's festivals, because he's one of (if not the) best cooks in town. This means he's friendly with a lot of the 'hired help' around town. Laborers and the servants of the few rich people in town. This is where a lot of his Intel comes from--because people aren't often careful what they say around servants, and servants love to gossip.

Threats

The Keepers have multiple threats facing them...

Druids of Yester Hill

The Druids are directly assaulting the winery, trying to destroy it in the name of their 'god,' Strahd. In my game, Strahd isn't terribly committed to seeing the winery destroyed. Even if it is, he'll see it rebuilt (eventually) and make sure at least the two gems are recovered and put back. After all, it wouldn't be fitting to invite adventurers for dinner and not have wine to offer them.
In a straight fight on open terrain, the wereravens would slaughter the druids. Blights lack ranged attacks, and the longest range magically-damaging attack the Druids possess has a range of only 30 feet. The Druids could simply fly up out of range and pelt the Druids with crossbow bolts (they'd be firing at Disadvantage, but the Druids couldn't retaliate). The problem is...the druids live in the forest, so the wereravens don't get their ideal engagement.
After the druids stole the gem, the wereravens failed in their assault on Yester Hill because they had to land to search for the gem, and Druids, Berserkers, and Blights would pop up out of the 'graves' to grab at them every time they tried, then go back below ground for cover whenever they took off.
The winery then fell because an entire horde of blights attacked and the 6 combatant wereravens simply couldn't kill them quickly enough to stop the winery from being overrun...and to clear the winery once it was breached, they'd have to fight on the ground...at which point, they could be overwhelmed by the sheer mass of the blights and, as mentioned up above, later killed in other ways.

Baba Lysaga

he wereravens are normally capable of fighting off the Druids, but fighting Baba Lysaga is suicide. Luckily for them, she doesn't move against them directly. Against her Scarecrows, they deal with the same issue they have with the Blights. The scarecrows can't harm them directly, but their 'Terrifying Glare' can paralyze a wereraven, causing them to fall out of the sky (they aren't immune to fall damage), and can then drag them off to be killed in other ways...or killed by Baba Lysaga directly (she has plenty of access to magic damage).
The wereravens know where she is, they know she has one of the winery gems, and they know her hut started walking shortly after she stole it (and they are capable of putting 2 and 2 together). At this point, their goal is to try to steal the gem back, but they aren't at all sure they can pull it off. They would love to see Baba Lysaga killed, but know they can't do it themselves and won't throw adventurers at her if they don't think they have a chance to win.

Ongoing Investigations

Apart from their direct enemies, there are things happening around the valley that the wereravens are either directly aware of and should be acting to counter as best they can, or are trying to figure out. The wereravens are the only real organized, active force for good in the valley. There are individuals who are good, but the wereravens are the only organization.

Werewolves

The wereravens are aware that the werewolves are stepping up their game and abducting people to turn. They are largely unable to intervene as they can't really hurt the werewolves any more than the werewolves can hurt them...but the werewolves are stronger than they are, so if it came to a grapple, the wereravens would lose.
By the book's lore, the werewolves are snatching kids from outside of Barovia, so the wereravens can't really interfere with the kidnapping attempts, since they can't follow the werewolves through the mists.
At this point, they are primarily observing and gathering all the information they can, and are hoping to find some skilled individuals that they can direct towards them. They are likely aware of the recent schism that happened within the pack.

Lady Wachter

In my version of things, Stella Wachter (The kitty-girl) used to be friendly with the Martikovs--she used to help babysit Brom and Bray. On top of that, Nikolai and Karl Wachter practically live in the taproom when they aren't causing trouble (one minor adjustment I made is that the one place in town they behave themselves is inside the inn--Danika has threatened to permanently cut off their wine if they cause trouble for her). As a result, the Martikovs know a lot about Lady Wachter's aspirations to take control of the town (though they don't know any of her secrets).
They are, of course, aware of the history of her family and the family's close ties to Strahd. That, coupled with the way that Lady Wachter kept sending Stella back to Victor even though her mental state was clearly degrading has made the Martikovs rather severely dislike her. However, they won't actually sic adventurers on her without proof of wrongdoing, but they may ask them to investigate. Especially if Wachter seems poised to take over the town (If liberated, the Martikovs are an extra resource that will take in Stella, then will begin actively searching for a way to restore her mind.)

Baron Vallakovich

The Keepers are aware of the excesses of Vargas Vallakovich. The Martikovs take it upon themselves to smuggle food and drink to the people he locks up in the stocks, and try to diffuse anything that may set him off. They are not thrilled that he is in power and (because of what happened to Stella) are not overly fond of Victor either. They would love to see Izek go away. However...The Baron and Izek may be the lesser of two evils, as compared to Lady Wachter. It's a hairy, complicated situation, and the Keepers are trying to gather more information before they consider actually doing anything.
Ultimately though, their goal is to fight Strahd, not risk blowing their cover to deal with a political power struggle or revolution within the town. They will protect people as best they can, but would prefer to stay out of politics.

The Abbey of St. Markovia

The Keepers are aware of the mongrelfolk in the Abbey. Easy enough to spot some of them on a simple flyby. However, they appear relatively benign, so they more-or-less have that listed as "things to worry about some other day."

Argynvostholt

The Keepers are aware of the revenants that live there and know a little bit about the history of the place. They may be aware that the revenants both kill Strahd's servants and people who try to kill Strahd. The revenants and the phantom warriors that fight at their command are dangerous, however...so the wereravens keep their distance.

Tsolenka Pass/The Amber Temple

Too cold for ravens up there. All the Keepers have are rumors. They know about the Roc of Mount Ghakis, though.

Khazan's Tower

They may not be aware of all of its properties--but know that it suppresses their shapeshifting if they get close to it, so they avoid it. The wereravens rely on their ability to turn into a bird and fly away if they get in danger, so investigating a place that forces them back into human form and keeps them there is dangerous.

Controlled Territories

• Blue Water Inn