I've got about, oh, a dozen concepts floating in my head.
Idea number one is a shapeshifting individual. He is capable of altering his body in many ways; he started by just mimicking animal life but quickly expanded his repertoire to creating unique forms by mixing up traits from several animals at once. His powers are instinctual, and help him adapt to a new form within a few hours of his first attempt, but before that he can barely stand up. He is capable of manipulating his mass, allowing larger or smaller versions of forms without difficulty, but he is not yet capable of engaging a giant eye-to-eye or taking on a microbe with his bare fists. It seems, however, that the more he shifts the stronger his powers get, like exercising a muscle, and there is no indication of just what his limit might be. His given name was Sam, but no one is really sure if that's short for Samuel or Samantha.
In truth, Sam was raised male by a loving nuclear family in east coast US suburbia; two parents and a younger sister. His family had the usual bucket of problems, but nothing extraordinary. When Sam's powers began manifesting at the age of seven, things changed. His parents panicked and called in doctors, specialists, psychics, veterinarians...the chaos lasted a year after which, finally, it became clear that Sam had abilities, and that was all that anyone would ever be able to figure out. The best explanation given was "Understand this, you will not. Powers he has. Control he must learn. Your money to me you will give now?"
Once the panic finally settled, Sam joined the superteam in order to get the training he needed. His parents at first treated it like a daycamp, then a school, then a boarding school as the years went on. His sister, on the other hand, was maddeningly jealous of her brother's abilities and tried to monopolize her parents' attention to compensate. With Sam spending most of his time away from home, it became easy for her to achieve that goal, and she grew up a spoiled little brat.
Sam responds to stress by altering his form to whatever seems least likely to be subject to that stress. A cat won't fear small places as much as a human, a hawk won't be afraid of heights, so on. He has an aversion to cleanliness as a proper teenage boy should, but in the event his uncleanliness results in disease he almost always has some kind of emotional break. Current theory is that his unique anatomy doesn't play well with other creatures and expresses its ire to the mind by altering hormone levels. Or maybe he just really hates being sick.
Sam's experience with sex is long and detailed. His talents make physical attractiveness a trivial accomplishment, and he is able to pick up a one-night stand any night he pleases. As alluded to before, he does this as a man or woman as his whim takes him.
Sam's dark secret is that he is lonely. He can be the life of a party, he can make a few phone calls and have an intimate meeting with anyone he wants, and he has comrades in his teammates, but his desire to master his abilities, the abilities that so terrified his ever-more estranged parents, has made it impossible to form any true friendships.
Sam gets going when he's told to get going or when he's really bored. Any excuse to show off is taken without hesitation.
Sam is not terribly open; he wants to please people so any suffering he feels is subjugated by him saying what he thinks others want to hear.
Another idea is the intellectual who manipulates technology to suit his needs, possessing about fifty different gizmos on hydrodynamic servomotors that spring out of his ass at a moment's notice. This type of character is contingent on the technologies available to him, and his effectiveness is dependent on the technologies available to others. As such no progress can be made on this idea without information of that sort.