About me: In the past, as "Frederick", he seemed to be a simple-yet-aloof scientist that was naturally a loner. He was content with having only two friends, while not bothering to compromise his own integrity to appeal to others around him. Upon becoming "Sol Badguy" however, most of his prior unruly traits, such as his gruffness and his jaded mentality, became much more pronounced.
Sullen and unrefined, Sol only speaks to others when absolutely necessary. Consequently, he is not very good at expressing himself, and tends to compensate by bullying people into line or simply steamrolling them. A lone wolf with a belligerent streak, Sol is actually an extremely intelligent individual who doesn't tolerate nonsense and gets "grumpy" whenever he cannot explain something with logic. He is also quite stubborn, never allowing the opinions of others to dissuade him once he has made up his mind. Sol is seen by most people as crude and self-centered, even those who might call him a friend.
Conversely, Sol hates working any harder than he has to, which means he rarely puts much effort into anything that isn't related to his goals, and is often irritated by having to deal with opponents. Sol's dislike for effort is even reflected in his fighting style: his moves are unrefined, but powerful so as to finish enemies with a single blow. Said laziness even extends to how he raised Sin, putting him through the most basic forms of education and rarely explaining concepts in detail.
He may cynically mock Ky's "justice", but Sol is utterly dedicated to avert further tragedy, even as he refuses to acknowledge it, and his actions prove that he is quite compassionate in contrast to his tough, anti-social persona. He spares the lives of Solaria, Testament and Dizzy, raises Sin at the behest of Ky and Dizzy, swears to save Elphelt, comforts Ramlethal, and even offers a prayer for Justice's soul as she dies.
Pre-Overture, Sol roamed the world alone and kept everyone at a distance. Bearing the guilt of working on the Gear Project, humanity's near extinction, Aria's death,and the burden of That Man's betrayal and being a "monster", Sol dedicated himself to eliminating all Gears and to exact his revenge on That Man. However, his clashes with other characters slowly begin to change his outlook, as does reluctantly taking Sin as his ward. In Revelator, Sol admits to Ariels that the world where he kept himself distant to others is worth saving, and finally acknowledges that despite what he is and his fear of the future, he is no longer alone in the world.
Despite his often composed demeanor, Sol expresses a nervous attitude whenever his connection to Dizzy is discussed. He is shown feeling embarrassment and is eager to avoid the issue, rather than acknowledge the possibility that he may be Dizzy's father.