MARK AND VANCE COSTNER: When the Costner twins (fraternal) were born to Aaron and Delia Costner, amidst severe complications, it was obvious the two were mutants. Vance was large, and born with a full head of white hair (which, as he grew, darkened in stripes, until by the age of two he had a zebra head, dark brown and white stripes), and Mark was severely deformed, exceedingly small and fragile-boned to the point where they did not think he would survive childhood.
As the boys grew they were very close. Mark, as he got older, not only survived childhood but proved himself to be highly intelligent, hyperactive and driven. Drugs worked on him oddly; he later was tested and shown to have a healing factor that worked specifically against poisons and drugs of any kind, negating or transforming their effects. Vance was very good-looking (they dyed his hair solid brown), athletic, and popular, but no supergenius. Instead of the brothers resenting each other's gifts, however, they relied on each other; Vance would beat the crap out of anyone who taunted his crippled little brother, and Mark demonstrated patience only when it came to tutoring Vance or fast-talking teachers to get Vance out of trouble. Even when they got old enough for girls to enter the picture, and Vance got them all, the boys managed to remain close, though no one could say Mark was exactly happy about it.
In his early teens, Vance started disrupting electronic equipment. Radios and televisions would fuzz out around him. Disks and tapes would die. And his strength, always high, began to increase. A bit of judicious experimentation by Mark revealed that Vance drew in and absorbed EM radiation and converted it to strength. He learned, under his brother's tutelage, to expand or contract his field, so he could watch TV and handle videotapes without problems, or black out an entire city street to gain the strength necessary to lift a car.
When the boys were 16, their mother-- frail since their birth-- was diagnosed with severe kidney and liver damage. Nothing could be done about her liver, aside from put her on a transplant list, but it was considered that Vance might be able to donate her a kidney, which he was happy to do. (Mark was too small to be considered.) So they tested Vance for compatibility-- and found out he wasn't Delia's child.
Aaron Costner had been present at the birth of his twins. He'd seen Vance's large size at birth and thick head of white hair. There couldn't be two babies who looked like that. Yet Vance wasn't Delia's child, or his either-- and Mark was. Moreover, Mark, who had assumed he was a mutant all his life, learned that he really wasn't. Technically, Mark was an altered human, someone whose powers had catalyzed in response to some sort of chemical alterant. Chemical analyses on Delia's kidneys and Mark's bones revealed that Mark and Delia had been exposed to some sort of toxin, Mark pre-natally, and it had catalyzed Mark's latent potential for power, not quickly enough to prevent it from deforming him but quickly enough to keep it from killing him. Vance, however, showed no signs of exposure to the toxin at all.
Mark joined the CIA as an analyst; Vance joined the military as an open mutant, and got a few assignments having to do with his power (jamming enemy communications mostly, though he did get to beat the crap out of Polaris once). But when anti-mutant prejudice forced Mark out of his job (the idea was that mutants couldn't get high security clearances; since Mark was outspokenly for mutant rights despite technically not being one, and since he looked like a mutant and his brother was a mutant, he was always assumed to be) and Vance's tour of duty ended, the two went into practice together as private detectives, with Mark as brains and Vance as brawn. Privately, the two are still on a case of great importance to them-- finding out who poisoned Mark and Delia (who has since died of her exposure, 20 years after being poisoned), who Vance is really related to, and what, if any, connection those two things have.
Sooner or later they're going to get somewhere by working the angle of their mother's ob/gyn-- a fellow by the name of Milbury... :-)
(BTW, as an adult, Vance no longer dyes his hair. But he's still obscenely good-looking, and so, despite being an open mutant, he still gets girls. Mark has managed to win a few over by sheer force of personality, himself.) Creator: Alara Rogers.