(As usual, there are spoilers in this post.)
I seem to be saying this a lot, but this is a difficult one. I wouldn't say that I have a favorite "minor" character. I have a few favorites:
I really like how Prim grows and matures in both Catching Fire & Mockingjay. She's seemed so mature for a 13 year old.
And lastly, Finnick. Unfortunately Finnick hasn't been cast yet (or if he has, it hasn't been announced publicly yet) so I don't have any photos to post, but I really do like Finnick a lot. I like both his serious and humorous side.