I started to type a reply and then found your comment did it for me. He is flawed- he has a temper he tries to control and mostly succeeds, but there is a lot of anger even before the games. He knows that he's good with words, and once he realises that he tries to use it to his advantage... and sometimes it backfires horribly. That's not a Stu trait. He has built up Katniss as somebody he was connected to, which is why he could believe her lies in the arena. He probably fell for her for a mixture of reasons, some good and some bad. I don't think it was a coincidence that the first thing he learnt about her was that she was the daughter of his father's ex, a nurturing and kind woman- a stark contrast to his own abusive mother.

As for why Katniss doesn't dwell, she has built him up as Somebody She Is Indebted To. That is the most powerful thing in the world to her. I think it's also the reason you don't see much of his family- they would illustrate his weaknesses, and she doesn't want to see that.