My favorite would have to be a young woman named Ren. She's a member of an "accidental" race (I write fantasy) created by another one of my protagonists, a race that's been persecuted by the humans since their creation. Having plenty of reasons to hate the humans, she obscures the features identifying her as inhuman and infiltrates their society, determined to undermine them from the inside. Then she meets the MC (she's a supporting character), learns more about what's been going on behind the scenes all this time, and decides to help him, all the while still hiding her identity from him since he's human.
There are a few reasons why I like her. The first is that she's incredibly conflicted--background activity or not, the humans are genocidal racists who nearly wiped her people out, yet her own morality is telling her to help them. She can't tell anyone who she is for obvious reasons, and she can't even divulge she's female, as women are not allowed into the group she's infiltrated--a detail that leads into my second reason. Since she can't speak for fear of giving herself away (she is not low-voiced for a woman), she has no dialogue, presenting a rather enjoyable challenge as I'm forced to find other ways to let her express herself.
Second place might have gone to my namesake Fenris (yes, the wolf from Norse mythology, adapted to the story), but I've actually gone back and revised a lot of his personality and purpose, so he's a little too uncertain right now for me to say for sure. Instead, I'd pick a 'person' whose working name is Symphony. 'His' plight is somewhat similar to Ren's in that he can't speak, but this time it's because he can't grasp language yet. He's essentially the manifestation of the world's spiritual energy who tries and consistently fails to understand the societies and cultures of the people living there. As words are beyond him, he uses music and sound to communicate emotion, hence the name given him by the people. Like with Ren, working with Symphony is...interesting in an odd and challenging sort of way, but it's not unpleasant.
Hi, my name's Fenris. I'm a thousand-year-old monster who's broken free to destroy the world. Your kids will love me!