Do you know good anime characters who have "absolute immortality"?

Saitama? Though I am not sure about the “not aging” part.

Maybe Zen’ō, though I am not sure that he counts as a “good” character.

Garlic Junior is immortal, because he got his wish granted by Shenron, but he is evil.

Galactus. Thor.

Alucard from Hellsing Ultimate
C.C. from Code Geass
Zamasu
Zeref from Fairy Tail
Aizen from Bleach
Naraku from Inuyasha
Kaido of the 100 Beast from One Piece
Elder Toguro from Yu yu Hakusho
Ban from Nanatsu no Taizai
Firo Prochainezo From Baccano
Hidan
It’s funny how most of these characters get their immortality through some kind of god, not out of their own effort or through some weird scientific discovery