Shinnosuke

Shinnosuke and his grandfather both live in the forest of Ryugenzawa, where they protect the local villagers from the giant animals that they once kept as pets. Years ago when Akane was vacationing with her family in Ryugenzawa, she got lost in the forest and was attacked by a giant platypus. However, she was saved by Shinnosuke. Some time later, Shinnosuke was near death by his grandfather, who took some of the Water of Life and fed it to Shinnosuke. Shinnosuke survived, but developed a dependency on the Water of Life.

Years later, in the present, Akane once again hears about the forest of Ryugenzawa in a television advertisement asking for help in combating the increasing numbers of monsters and remembers Shinnosuke saving her. In an attempt to pay back Shinnosuke, Akane travels to Ryugenzawa to find him. In the process, Akane is attacked by a giant creature and is saved, just like when she was young, by Shinnosuke, who still bears the marks on his back. Although he is now very forgetful, Shinnosuke invites Akane to stay with him and his grandfather. Akane later cures Shinnosuke of his dependency on the water of life by applying a special moss to his back.

(Source: Wikipedia, edited)