Severus has always known, without a doubt, that Harry Potter is his biological son, but still resents him and has no interest in forming a relationship. After Harry has found out, he decides to try and win his father over.
Why Harry chooses to pursue the relationship rather than writing Severus off is up to the author, but keep it in character.
Canon placement up to author, but if writing an AU don't change lots of non-connected things just for the sake of it.
- Severus is very stubborn and can't change his heart too quickly or easily. Lots of angst on Harry's end needed before any flicker of affection received
- Severus genuinely hating Harry, protecting him only out of a sense of duty, absolutely no affection for him to begin with
- Character bashing
- Severus resents Harry for more than just being the reason Lily died
- Harry's friends help come up with ways to win him over
- Fred and George making bets on the outcome
- Harry trying to be "more Slytherin"
- Harry's friends try and persuade him it's not worth it
- Massive row between Sirius and Severus
- Harry's self-esteem reaching an all-time low
- Abusive Dursleys
- Severus finds out about the Dursleys, he takes action but still can't stand Harry
- Severus sees himself in Harry which adds to his reasons to hate him
- Some point (very late) in the story, Severus almost loses/thinks he has lost Harry and it's a turning point
- Harry embarrassed by the knowledge that his real father hates him so never tells anyone, puzzling his friends why he's trying so hard to get into Severus' good books
- His friends find out by accident
- Harry makes studying Potions his top priority and manages to bring his grades up
- Somehow Harry knows about Severus being his father before he starts Hogwarts, and asks to be Sorted into Slytherin to be near him
- Or he doesn't find out until much later, when he and Severus already have a long history, and Harry has to do a lot of work on his own feelings before he approaches Severus
- Severus never told Albus that Harry was his son because he knows Albus would try and persuade him to tell him the truth/form a relationship with him