The Immortals of Doctor Who
(spoilers for all seasons of Doctor Who and series 1-2 of Torchwood)
this is theoretically written for non-whovians to understand it if you’re so inclined
Captain Jack Harkness
How did we get here?
After he was shot and killed by a Dalek, he was revived by Rose Tyler, who at the time was transformed into a nearly omnipotent being. Unable to control her powers, she accidentally turned him into an immortal being. [1]
Now what?
Jack can die, but he always revives himself. The refractory period is unclear, but it doesn’t matter whether he is killed immediately after a previous death or not.
Since he is a fixed point in time, it isn’t always that he can’t die; to die he theoretically has to have existed in all points of time before he could die. However, we know that he most likely becomes The Face of Boe and dies after over five billion years, which while quite a long time is not nearly the length of the universe. He can also double over on his timestream as long as he doesn’t meet himself, though the effects of said meeting are unknown.
Doctor Owen Harper
How did we get here?
Owen got shot to protect Doctor Martha Jones, and was revived temporarily to give the passcodes to sensitive Torchwood information. He was bonded to the embodiment to Death, which he eventually fought when he didn’t fall dead again. Toshiko (his co-worker) told him that there was residual energy from the experience that would eventually fade away, though the time frame was unknown to them. Until then, he was perpetually dying, which he teased Cptn. Jack Harkness about, telling him that they were quite the pair (he was always dying while Jack was always living).
Now what?
Owen can get hurt, but he cannot heal or feel pain. Since his body doesn’t function, he does not need to shower or any similar cleansing rituals. He must keep himself exercising to not go into rigor mortis, but he doesn’t tire out or need adrenaline/food/water to keep moving. He gives off no body heat and cannot forcibly expel breath, though he can talk.
Ashildr (Daughter of Einarr)/ Me (Mayor, Lady)
How did we get here?
After an adventure with the 12th Doctor and Clara Oswald, Ashildr died and was laid to rest as the only casualty of their fight against the warrior race the Mire. Infuriated and frustrated at his inability to save her, the Doctor used a medical path from the Mire to resurrect her and gave her another to give to whoever she wished to keep with her.
DOCTOR: It won’t stop, the repair kit I put inside Ashildr, not ever. It’ll just keep fixing her.
CLARA: Well, good.
DOCTOR: I’m not sure, but it’s entirely possible she has lost the ability to die.
CLARA: The ability?
DOCTOR: Oh, dying in an ability, believe me. Barring accidents, she may now be functionally immortal. [2]
Now what?
Ashildr is now immortal until something can manage to kill her. She is seen at the end of the universe, having outlived all other immortals (including the Time Lords and the Sisterhood of Karn), though the implication is that the end of the universe would be enough to finally destroy her. She is taken away, however, and continues to travel for an unknown amount of time with Clara Oswald in a stolen TARDIS through time and space.
DOCTOR: You’re immortal, not indestructible. You can be hurt, killed even. [3]
Clara Oswald
How did we get here?
After Clara Oswald, travelling with the 12th Doctor, resigned herself to a Raven curse, which was tied to her soul, the people of Mayor Me’s alley witnessed her dying and falling to the ground. However, the Doctor wasn’t ready to let her go, using Gallifrey and his power there to fracture into her timeline the heartbeat before the Raven hit and killed her.
Now what?
Since Clara was not alive when she was frozen, she was only on pause. When she travelled with Ashildr, it was known that she would eventually return to Gallifrey when she felt like she was done with her travels. Until then, nothing could theoretically hurt her, as her death at the Raven was a fixed point in time.
The Timeless Child (The Doctor)
How did we get here?
Once, a space traveller found an interdimensional child at a gateway to another dimension or dimensions (not indicated to be the same as the universes that are shown to hold the Tylers and the Metacrisis Doctor). Taking the child, she raised it as her own, but couldn’t save her forever-- the child fell to her death but regenerated into another version of herself. After extensive experimenting, she found a way to harness this regeneration energy, creating the race of the Time Lords on Gallifrey. The child’s memories were wiped and they were re-integrated into society, eventually known as The Doctor.
Now what?
It is uncertain how many rules of regeneration apply to the Doctor. While Time Lords are vulnerable immediately before regenerating when injured, no one is sure how the Doctor would react. The only instance of this happening was via a duplicate that did not understand their biology fully and could only shallowly recreate the effect. Their regenerations seem to be limited after their memories were erased, but the amount of cycles or individual regenerations they possess after the Time Lords gave more to them is unknown (if they even gave any at all versus just covering for the unlimited potential).