User blog comment:Jopjopjop/Volume 19 Spoilers/@comment-2143738-20141206004048/@comment-16281343-20141206011426

It's Tosuka.

~Okay. I haven't reached that part yet (I'm still at the start of Life. 2) but saying Tosuka is his girlfriend is pushing it. I know the spoilers said the word girlfriend and Tsubaki treating her as a rival but she treats Issei as a rival too. The word girlfriend can be used too to make a point about Kiba not having any involvement with other females.

From what I've gathered, it was just them (Kiba and Tosuka) overly happy about meeting each other again. They hugged each other and that's understandable with what happened to them.

TL;DR : IMO, Tosuka is not Kiba's girlfriend.