DR. REBECCA JANSEN/LIGHTMAN: Rebecca (aka Rebecca Jansen, Lightman, etc) is an immortal mutant. (No relation to Tiamat, or anyone else, so far as she knows.) This is her only power. She's not an External (there's no such thing in XXY), and she doesn't have a healing factor per se, but when killed, she comes back from the dead regenerated, and when being starved or suffocated, she suffers pain but does not ever actually quite die. She was born to a Jewish family in the Netherlands in 1604, but she's been all over Europe, adopting multiple identities throughout her lifetime, frequently passing for male so she could have freedom of motion (and so she could have female lovers; Rebecca is a lesbian). Throughout most of her life, she worked to help and heal others, learning herbcraft, midwifery, surgery, and finally modern medicine.
She was, in fact, back in her original homeland, working as a doctor and openly female (she used the name Rebecca Jansen then), when the Nazis invaded. Rebecca joined the Resistance. The Nazis caught her and tortured her. Up until then, Rebecca had tended to see atrocities as horrible things that happened to other people, but nothing that endangered her; it was her desire to help other people that drove her, not her fear for herself, since she was immortal. From the Nazis, Rebecca learned that there are fates worse than death, and that immortality doesn't convey invulnerability. She broke and told them everything, not only betraying all of her Resistance contacts, but also revealing her own identity as a Jew. Her non-Jewish Resistance contacts were sent to prison, but through one thing and another she was sent to Auschwitz, where she befriended and mentored the young Erika Lehnsherr. (Rebecca didn't reveal her nature as a mutant to the girl, and neither Erika at that time nor Rebecca had any idea that Erika herself was a mutant.) In the death march from Auschwitz toward the end of the war, Rebecca was shot and left for dead (actually she was dead, but she got better.)
After this experience Rebecca changed. No longer willing to merely be a doctor, trying to help what few people she could, she decided to go into medical research, and try to help all of humanity at once. She became obsessed with aiding humanity, and atoning for her own sin in surviving atrocity, by finding the key to granting the immortality she has to everyone else. However, her obsession is not a mad one; Rebecca is a Sane Scientist, dedicated, driven, but not unethical. As a Jewish female doctor in Auschwitz, she saw Mengele's work firsthand, and would not accept the realization of her dream if it required her to sink to that level; she's an immortal, and if it takes a dozen human lifetimes to find what she seeks, so be it. She is a well-known medical researcher under the name Rebecca Lightman when X-Factor begins, but most people don't know she's working on immortality. Ironically, both she and Carolyn Xavier are well-aware of each other and have met several times because of their shared activism in feminism, without any idea that they have a friend in common (who of course by that time is no one's friend, a fact Rebecca will learn to great pain...)
Rebecca has an ironclad ethical system, but it's not the same as, say, Carolyn Xavier's; she's got close first-hand experience with environments where there are no ethical choices possible, and one must choose the lesser of evils. Thus, certain things just do not strike her as morally wrong (like seducing a 15-year-old protege in a concentration camp, her logic being that if her friend is old enough to work like an adult and die like an adult, she's old enough for whatever small pleasures adults can give each other; this is logic that would appall Carolyn, though Erika herself is untroubled by it.) She's capable of emotional ruthlessness, of being terribly cruel to people in a cause she considers good, but she can't kill, and she would never experiment on sentients without informed consent. As far as sexual preference goes, she's attracted exclusively to women, but she is not into children; the thing with Erika was because they were in hell together and she saw Erika as an adult, albeit a young one (as an immortal, the distinction between 15 and 18 seems trivial to her anyway unless it's backed up by personality, which it usually is in most people.) Generally she goes for short-term relationships so she doesn't have to see people age and die, but this saddens her and she longs for something that could last a lifetime. She's infertile (just because she's a lesbian doesn't mean she hasn't slept with men, lots of them over the course of 370 years...)
Her power is immortality. When she suffers an injury traumatic enough to kill her, she will die and then regenerate completely (thus, if someone hacked her arm off, it would not grow back unless they then shot her in the heart.) She handles deprivation and illness by getting weak and ill and suffering, but never actually dying of it (thus, if you put her in an airless room, she'd choke and feel a need for air, and her mental processes would become impaired from lack of oxygen, but it would never actually kill her; if she caught a fatal disease, she'd get really, really sick and then recover.) She doesn't age, and looks about 30. She doesn't have enhanced senses or ferality, and if you hurt her, it won't heal especially quickly unless it's a fatal injury. She is petrified of torture, given that she can't die of it, and being buried alive (it's happened frequently). Creator: Alara Rogers.