Ambassadors and Allies

The rulers of some nearby culture know about you and your new stronghold and demesne, and they want an alliance. That, of course, may take some time and negotiation, so in the meantime they’ve sent an ambassador! Ambassadors allow you to purchase units from their ancestry as though they were friendly. If you are ever awarded units you didn’t buy—by rolling on a follower chart or adding a keep to your castle, for instance—then the new units can be from the ambassador’s ancestry if you wish.
An ambassador is a representative not only of another species, but another government. An orc ambassador isn’t a representative for all orc-kind—they represent a specific tribe of orcs. Their goal is to eventually sign an accord, either officially or unofficially, but for now they’re happy just having a representative in your court who can advise you and ideally pave the way for a more lasting alliance. These are rewards. Not enemy spies.
They are diplomatic opportunities for the player, part of the reward for building a stronghold. Players may be suspicious of this stranger who arrives promising military aid cheap, but you should encourage them, even tell them explicitly if necessary, “This ambassador will work with you.” Of course, this could present some amazing narrative and roleplaying opportunities down the line if events unfold that drive a wedge between the PC and their ambassador, but as long as this is a natural development of events, and the player understands this, it should be fine.
ALLIES You might end up rolling on the Special Allies chart based on your alignment and attract something really interesting like a black dragon or a sphinx! Or a stone giant! Unlike followers, allies don’t actually work for you, as they are often way more powerful than your character. You can’t give them orders and expect them to obey. But they are interested in helping you. They are local personalities who’ve been watching your development, they approve, and they arrive interested in doing you a favor.