Which seems very safe for her...
The best thing to do is ignore him completely. Fighting with him will just reinforce a social connection. He is probably stalking her because she wears anime stuff and he likes anime so she must be his soul mate. Any acknowledgement (positive or negative) that he exists is going to give him hope or piss him off. Neither is a good idea. Like a big scary dog, you want him to just get bored and go away.
If he doesn't, if he keeps following her around, the school, police, parents, and friends need to be informed.