Stephen Leeds was an American on Earth.
He was a highly intelligent person, but was unable to function at such a level. His brain created aspects, which were compartments of his brain as hallucinations. Only he can see them, and each is an expert on one, or possibly multiple, topics. Stephen must learn the information he needs from these aspects, even though he already knows it. Normally, the aspects can't affect him at all physically, but during the events of Legion, J.P., an aspect who 'was' an ex-special forces, is able to somehow 'teach' Stephen how to roll our of a car the proper way. Stephen is able to skim through a book, or listen to a tape, and though not focusing on it can create(unconsciously) another aspect with that knowledge. However, since Stephen is unable to learn from them without being close enough to physically learn from them as if they were human, he can be separated from them if an enemy knows how his mind works. In Legion: Skin Deep, Z-- uses this to make Stephen lose them. She pretends that they are getting out of a car, and after Stephen lets his aspects go first, drive off. However, this limitation is likely only a mental barrier, since once two of the aspects have a phone(also imaginary), they can call each other and relay information to Stephen. Stephen has a large mansion, with one room for each aspect, since when they are uncomfortable or cramped bad things can happen involving nightmares and aspects disappearing.