Yami ni Tooboe, Mune ni Toge
Complete | naono bohra | 2000 released
This mangaka is always so creative! A human with a soul bond to a powerful demon is reincarnated into a 17yo high school student who cannot get close to other people, or they end up gravely injured. Also features the bonded demon's hilarious older snake brother and others!
Kimi to Kore kara