There's like... a period of around 2 or 5 years in between that flashback and the present time in the arcade that doesn't really make sense xP or in fact, it makes it sound like she didn't hang out with Sonomi for many years and the only thing she had was a keychan, and the excuse is that she was always surrounded by people but I think Sonomi could've done a way better job at being friend with her, you know like... not leaving her alone? or at least noticing, of course, the thing with pride and her rude attitude didn't help at all.
As I thought, I really can't like Sonomi after all -.-