From what I know.
Yuri: Lost her little sisters long ago, and regrets that day since she could not save them. Is a very leader-like person that is very good at it, loves guns
very friendly, had a spark for a guy while she was still alive, and it somehow continued on to the Afterlife, where she had to stop him from creating these creatures that make you into an NPC. Has a thing for either Otanashi/Hinata (Hinata mostly) lived a very boring school-life (hence her expression in the flashback)
Judging from that alone, she might have just got into an accident like you said, and the regretting thoughts about losing her little sisters brought her to the Afterlife.