"Standing in the ritual circle was my fellow comrade at-arms. When we both agreed to enter this war, I didn't know what to expect. He was always really good with a bow and I was decently good with magic. When the ritual began, his body began to glow a bright light and his eyes and mouth began to have radiant light pour out of them. Moments later when I regained my vision, he stood there. But something was different about him...and his bow..." -Charles Allemand. Lekronese War Mage. 399.  

Vessels of Heroes

When a person becomes a pseudo servant, the most dramatic change that happens to them occurs within their very soul. The Holy Grail is what determines on whether someone gets to become a pseudo servant and if the Grail approves, it promptly infuses the soul of a heroic spirit into the soul of the person. That person now shares the soul of the heroic spirit. The heroic spirits in question are ones that could've become an actual summoner servant, but due to certain circumstances, have needed to use another person's body as a vessel.  

Infused Souls

With a heroic spirit infused into the pseudo servant's soul, that person in particular can end up changing in more ways than one. A warrior who started off as being loud, brash, and rowdy, after being infused with the soul of a noble knight, may start to act humbler and calmer. However, because a chunk of their soul had to be removed and replace by the heroic spirit's, a part of their original character is also lost. An elf that turns into a pseudo servant may suddenly find out that they can't see as well in the dark, or a dwarf finds out that alcohol seems to affect them a lot more than previously.