At 2:32 it basically says it all. She's not really in the chair. This means, of course: she doesn't exist. The storyline of video is that the girl is presumably dead, and though the guy wants to forget her, he sometimes feels like she is still with him (hence offering her the ice cream at first, and then sadly realizing he must eat it himself). She still follows him around, sad that he's continuing on in life without her. He is making an effort to move on, but he wants so badly for her to still exist. When he runs back at the train station, it is as if he's saying 'I know you're there, I didn't forget you' and she knows she's been holding him back, so she finally disappears, crying 'wasurenaide' (don't forget me).
Right, in this case we can assume he can see dead people. Because only a brief moment later his friends turn to him as if to ask "who are you talking to" before he returns to his friends and she is not seen in the chair
OnlyMusic16 Actually now that I observe more I don't think he can actually see dead people. When he looks in her direction he smiles and nods but doesn't actually open his mouth in a talking way. I think he was just looking there and imagining her being there because he knew her/misses her as well, and coincidentally at the same time she turned to face him and smile.