Hide's mother died when he was 7, so he was basically raised by his neighbor(s). SO I thought it would be cool/interesting if his neighbor had a daughter, who he then became somewhat close to since they essentially would grow up together. Granted, his personality is consistent, and even though he's angrier now, he was still a rude meanie when after his mum passed. He would have picked on her, acted like he didn't care, perhaps even pretended like he hated her, but on the occasion he would show that it was just a front. Whether beating up kids that picked on her, or carrying her home if she scraped her leg by falling off a bike -cute things are cute. 

She'd be one of the very few people that have seen his more sensitive side, and I don't doubt that despite his attitude most of the time she sees through his anger, and knows he's just been affected negatively because of his complicated family dynamic. However, he's always considered her a sister -even if he might not call her one to her face, and undoubtedly she feels the same. 

FC is open since I don't like telling ppl who to use, as well as personality - so long as she can tolerate his tude lol. I'd like her to be younger, by a couple years at least. I pictured her being in high school still, but I could work with college freshman.