Yayoi's been taken from post-sabraather which was a very brief dressing room that took characters from Sabra and Aather and threw them onto teams together. Yayoi came from Sabra for the purposes of that DR, so her history is Aather's darker cousin. She grew into more of a healer character there, gaining healing and support type materia and eventually some epic surgeon skills after a huge plot event. She's been involved in plots to kill gods, though she was pulled out of Sabra and dropped into Sabraather before anything came of it.

In Sabraather, she and one of her other teammates were placed together with about half of Coral (Kit, Shark, Cub, Dragonfly). And they got to play in a cannibal game. :') Two members from each team became cannibalistic monsters that needed to eat or they'd eat their own team. There was also a member of each team that was a heart, and if they were killed the team lost and the cannibal monsters went berserk. Dragonfly got eaten right away, so Coral lost it even though he wasn't the heart because Coral. And Cub went completely insane and killed Yayoi's other teammate, ate Shark's arm, and mangled Yayoi a little. And then later killed everyone but Shark in the middle of a round. And then an angry mob killed Shark too.

So. :') Yayoi snapped and developed some epic repression skills. She never admitted her teammate died, insisting he was only asleep the whole time. She flat out does not believe otherwise. A recent heartgame made that tendency even worse so she'll mentally play down injuries as well. Terrible stab wounds may only register as deep gashes to her. Nasty but not as dangerous as they actually are.

How's Vincent doing with Saffron overall so far?