If Gabriel counts then he's by and large my least favorite, because I don't care for him as a character, a visual design, and I can't personally respect what he and his game represent.
Aside from him? Hmm. That's really tough to say. Interestingly, I never got too into Leon Belmont. I think every other character in Lament of Innocence, save for Sara, is a more interesting character, even Medusa and the Succubus. And I'm not all that into his design or voice acting, either. I don't dislike Leon, per se, but if I had to choose, then he probably does the least for me. Soleiyu even captivates me more than Leon, interestingly.