A Puppet's Heart: Two Strings Attached Puppet

Kamen Matsouka, a Japanese graduate, becomes unsuccessful in his life. He works at a part-time job for three years, gets disowned by his parents, and has no relationship with anyone else. Kamen needs to have a purpose in life and wants to end things here and there, not until a burglar shows up inside the convenience store he is working at. He saves a girl and takes a bullet in his chest.
By the time he wakes up inside an unrecognisable cavern, a girl in a gothic dress fixes his limbs and body. Her name is Alice, a princess who has escaped the befall of her kingdom. She is the kindest woman Kamen meets in his life.
Kamen finally has a purpose. He vows to protect the girl from the treacherous kingdom with guns, modern technologies, and tanks at their disposal. But Kamen becomes a doll, not a real human being. The girl has created him to keep her company inside her isolated cave. He also wants to become human and become one with the girl. The two will embark on a quest to find this Fountain of Life, a potion that will grant an individual life. Since the girl is also a princess, Karmen includes himself in her royal political issues in girl's fallen kingdom.
In an unfamiliar world where magic and guns clash, will Kamen and Alice defend their empire and bring peace? What happens next is for you to find out…

Book Cover is not mine. The anime came from Orchis from Grandblue fantasy.