After escaping heaven and returning to earth, Charile Barkin (All Dogs Go To Heaven) was immortal so long as he kept his gold watch.
After being given half of a dragon's heart, King Einon (Dragonheart) was immortal so long as the dragon lived.
Utsuro (Gintama) remains immortal so long as he continues to draw from the Altana of Earth.
Kouka (Gintama) remained immortal so long as she continues to draw from the Altana of Kouan, which is why she died when she left.
Voldemort (Harry Potter) was immortal so long as his horcruxes weren't destroyed.
Amon (Starcraft) cannot be killed from any attack so long as his Void Crystals are still in place.
Asphodelus (Valkyrie Crusade) is a spirit that is completely immortal as long as exists souls in the land of the dead for her to feed of.
Hanatamago (Hetalia) is an ageless dog as long as she is owned by Sweden and Finland, due to the fact that her owners's longevity rubs off on her. If she were to be separated from them, she would become a normal dog and age normally.
Phantom (Marchen Awakens Romance) will never die nor age as long as the Zombie Tattoo curse remains on his body.