My companion is the strongest undead in another world

Yuma was an ordinary college student who was suddenly summoned to another world. Like the typical isekai protagonist, he was given powerful cheat-abilities, so he thought he was about to begin a fun life in the new world, but what awaited him was a cold and cruel treatment. He was captured, his body mutilated, as fear and intense pain ran through his body. And he was not even allowed to die, for despite Yuma's heartrending cries, his "immortality" ability continued to regenerate his body. Yuma was reduced to an experimental animal by the researchers who were fascinated by its potential. Finally his life began to change for the better when he met Eva, a beautiful girl also being kept in the facility!

(Source: Comic Regalia, translated) Translated with www.DeepL.com/Translator (free version)