I'm looking for a mature protagonist whether that'd be in terms of age and/or the mental capacity of the character.

I'm not looking for a specific genre, so don't hold back

The MC in Kiss Wood should be pretty old, since he has white hair and beard . I read it a long time ago, so i don`t remember his actual age.

Ooh~ although the basic story sounds cliche (of being suddenly in a mysterious location), however, maybe because of the protagonist, the story sounds so original, ill check it out ^^

Uhu, your request is a wide field. So just a few random suggestions from my lists:

josei:
Kimi wa Pet
Midnight Secretary
Usagi Drop
P.A.

seinen:
Billy Bat
Uchuu Kyoudai
Haruka na Machi e
Hito Hitori Futari
My Girl (SAHARA Mizu)

shoujo:
Conductor
Oooku
Piece

shounen:
Shibatora
Death Note

Arigatou gozaimasu ^^