Charlie B. Barkin (All Dogs Go To Heaven); via his watch
Einon (Dragonheart); via sharing half of a dragon's heart
Sephiroth (Final Fantasy VII: Advent Children); via Geostigma/Jenova Cells
Max Winters/Yaotl (TMNT)
Tom Riddle/Voldemort (Harry Potter); via Horcruxes
Melkor/Morgoth (The Silmarillion); via Arda itself
Live Action TV
The Man in Black (Lost); via Heart of the Island
Natural mermaids (Mako Mermaids); via their magic to the Moon Pool
Flint (Star Trek)
Alaric Saltzman (The Vampire Diaries); via Elena Gilbert's life
Freya Mikaelson (The Originals); via Niklaus Mikaelson's life
The Demon King (Doraemon: Nobita's New Great Adventure into the Underworld); via the red moon surrounding Magic Planet
Nations (Hetalia); expect China
Holo (Spice and Wolf)
Phantom (Marchen Awakens Romance)
Quon (Towa no Quon); remains absolutely immortal as long as his younger brother Towa remains sealed.
King Gilgamesh (Tower of Druaga: Aegis of Uruk); remains immortal as long as Druaga exists
43rd Floor inhabitants (Tower of God); remain immortal as long as they remain on the 43rd Floor
Hanatamago (Hetalia); is immortal as long as she remains owned by Sweden and Finland
Ulysses Bloodstone (Marvel Comics); via the Bloodgem
Users of the Infinity Gauntlet (Marvel Comics)
Users of the Cosmic Cube (Marvel Comics)
Ra's Al Ghul (DC Comics); via the Lazarus Pit
Koschei the Deathless (Slavic Mythology) via a Needle hidden in an Egg, inside a Duck, inside a Hare, inside an Iron Chest, buried under a Tree on the island of Buyan.
Liches (Advanced Dungeons and Dragons); via their phylacteries.
Omega (Dirge of Cereberus)
Scathach (Fate/Grand Order)
The Deathless Ones (Rise of the Tomb Raider)
Jacob the Deathless Prophet (Rise of the Tomb Raider)
Amon (Starcraft); via his Void Crystals
Psi Reavers (System Shock 2)
Asphodelus (Valkyrie Crusade)
Doppelgangis (Realms of the Haunting)
Greater Psi Reavers (System Shock 2)
Reaver (Fable Series)
Yuyuko Saigyouji (Touhou Project)
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.