Guys, tinfoil hat time. Hear me out... Summer lives on, inside one or some of the other characters.
There are flashbacks and hints of ER / hospital and that Nika saw summer one last time but cut off and didn't speak on it.
Amber: But not the last time you saw her?
Nika: *You slowly shake your head* I did see her one more time and-...
Nika connects with some people, like Bella, as if he had known her for so long, and she mentions that she feels the same when you go to the gym with her. Something happened with her twin sister, and she might have been involved, others like Sasha etc. might have as well. It's possible they all ended up in some huge accident (or victims of some kind of attack).
What if the reason Nika feels like he is connecting with them so easily is because... Perhaps summer's heart, other organs, are inside some of these others because she died and the others were able to be saved because of her. People who get donated implants often say sometimes things change, like they like eating something they never have before... It's a stretch on logic, but in this game-world, he could be "connecting" with her in this sense. Taking this logic in, besides the beginning prank pulled on her, it would also add on to why Nami feels some kind of resentment / jealousy towards Bella, if a part of Summer was inside her for example, but even if not, Sasha might be related in this sense. She certainly eyes Nika, a lot.
What do you guys think? XD