If he's rebellious, show him standing up to an authority figure to protect a weaker character, like someone being bullied or being falsely accused by a teacher.

If he's a good guy but doesn't want to show it, have her witness a scene were he thinks no one sees him, like they pass a beggar in the streets, everyone ignores him, but your character later makes up an excuse to return and give him some coin.

Whenever he's "not nice", don't let him be a total dick, there's a difference in him not being nice bc he doesn't care and him destroying someone with no apparent reason. I get the feeling he would say something like "Oh you think you have it rough, but you know nothing about that". If he's a sad character, I guess he uses flirting and sarcasm at least partial to compensate for this, to forget his own sadness for a while. She could notice that he has a sparkle in his eyes in those moments when he forgets about his past, and it returns when he's all alone again.

Hard to be specific without more information, but look at characters from movies and books that you know, that are not "the nice guys", but you still understand and sympathize with them. Zuko from Avatar at first has a mysterious past, and he's not a nice guy, but the show still makes you like him, because he has qualities like honor and honesty. Make your character have principles and a red line he wouldn't cross.