Theta

Theta Sigma, the interdimensional crusader

Physically identical to the Tenth Doctor in terms of build and features. Supposedly the same species (? alien life in the Isles is not unheard of, but Theta is unusual by all accounts of documented alien life among Ralakay's borders and beyond) and often makes snide jokes about being a "science experiment" and "freak of nature." He appears to have the same anatomy as well but is able to exert much more control over his body. The Doctors can all produce a bright energy on command that seems to be capable of sustaining most biological processes in dragons, beasts, and other animals (whatever they are) but Theta can not only weaponize his at will, vaporizing whatever is in its path, but his supply seems to be able to recharge indefinitely. Theta is essentially invulnerable, he can use this ability to heal catastrophic injuries that should either kill him or, according to the Doctor, require full regeneration (whatever that means). Such an energy expenditure renders him unconscious for an extended length of time and must be supplemented with massive caloric intake upon waking. He also has an unusual amount of physical strength for a dragon of his build and size.

Theta is bitter, sarcastic, and quick to anger. He's no stranger to blood and has no issue striking first and asking questions later. He seems to always have rage boiling under the surface, not unlike the Doctor, but he's far less adept at controlling it. Or maybe he doesn't care enough to. He is consumed by his crusade to stop a clan he calls the Silence, who he claims to be unspeakably powerful to the point of being able to manipulate not just consciousness but time itself. He once belonged to an army that hunted the Silence before it was decimated and he was sent fleeing across the multiverse (multiverse? he's one of the few uses that phrase like it's common knowledge). Now he seeks refuge among the Isles in hopes that the advanced technology in the Arcanist's Observatory could help him continue his currently one-drake war.

He can usually be convinced to help if there's a chance he'll get to blow off steam by spilling some blood, but he does have good hearts somewhere inside centuries of pain and torment. No one has seen or heard of the Silence, and Theta rarely talks about his past unless it's about some random, outlandish romp. But the many levels of pain in his voice when he does mention them eliminate any doubt that the Silence are no myth. He has suffered deeply at their hands, trauma that continues to carve deeper with each waking moment in spite of claims to the contrary.

In that way, he's not that different from the Doctor. But the Doctor tries to turn his pain into kindness, something Theta can't bring himself to do. Not for any malicious reason, he doesn't feel capable of kindness. Strategy helps keep his mind clear, as does the deep, ever present desire to undo the wrongs of the Silence.

He loved once, but his love has been missing for a while. He had a similar origin as Theta and saw the good in him no one else did.