Alucard, without a doubt (huge fangirl for about the last 15 years, it was he who was my introducion to Castlevania). Then again, if I look at LoS, Alucard is a Belmont, so for the purposes of favourite non-Belmont, I'll just consider the original timeline Allie. After him, I'm also a big fan of Soma and Joachim, who I'd have loved to have seen a return of. For background small but very important characters, I think that Lisa is a very powerful character in her own right.