Well, for one, Aimu is Yari's twin sister...so there's that.

As for Lulu, Yari & Aimu have known each other from when they were little and going to Japanese language school together, so Lulu is more like their unofficial sister/cousin if anything.

As for the other two?

Yari really only has his eye on one of them but has been too nervous to really tell her how she feels. After all, he doesn't want to ruin a long-standing friendship over something that might not be.

...sad part is the other party feels the same way as he does.

But yeah, that's how Yari ended up the sole guy in their group, and if you haven't figured out who he's down bad for?

Hint: It's not Irene.