He would forgive her, he was sure of it. As if it had ever been a question.
He'd forgive her for all the times he'd had to watch his best mate be crushed by her rejections, and for the subsequent hours of therapy that were necessary to calm him down and hours of pranking needed to cheer him up.
He'd forgive her for stopping all their fun over the years. Well, okay, not all of it, but a large amount. It had saved a lot of time spent in detention.
He'd forgive her for shouting at him, and being generally rude to all of the marauders, 'cept perhaps Moony.
(He'd forgive her for being biased towards prefects too).
He'd forgive her for making Prongs grow up and accept responsibility. At least they'd graduated now.
He'd forgive her, and this was a challenge, for hating quidditch and deciding that prefect duties were more important, making Gryffindor quidditch practises be subject to hasty re-scheduling at her insistence. He may even forgive her for the sleep lost to the bitterly early mornings. They had won, after all.
He'd forgive her for calling him 'Black' and frowning at her so much he thought she was stuck with the facial expression forever. He'd only thought that for a bit, actually, as she'd started rolling her eyes, and then she stopped altogether and started smiling and called him 'Sirius'. It was quite nice really.
He'd forgive her for dating Amos Diggory in fifth year, forcing him to practise some very difficult spell work to control James and simultaneously hex the git. It had come in pretty useful in charms anyway.
He'd forgive her for preferring cats to dogs. (If she changed her mind).
He'd forgive her for standing out from the beginning; eyes flashing, hair whipping around her, and continuing to best the Marauders. It just made it easier to spot the source of Prongs' misery and guide him away from her.
He'd forgive her for interrupting him over the years. Those broom cupboard fumblings weren't all that anyway.
He'd forgive her for making him care about other people, and challenge his comfortable (if self-absorbed) point of view.
He'd forgive her for changing them all.
But only, only, because she was Lily Evans, and she was funny and fiery and beautiful, and had just gotten engaged to his best friend. And only if she asked.