Petals of Reincarnation

Touya Senji is talentless. He's tried everything but he can't seem to excel at anything no matter how hard he tries. Then he meets Haito, a Kendo champion and... humanity's protector? She uses the Stem of Reincarnation to draw on the abilities of her past lives, but it only works when she slits her own throat. And now she wants Touya to join her?!

(Source: Mangamo, edited)