His exact background is unknown, it is known that Sol's real name is Frederick and he was one of the lead scientists of the Gear project, he is also the prototype Gear from over a hundred years before the events of the Guilty Gear games. Being a prototype he is immune to orders from Commander Gears. His headband is used as a Gear-suppressing device which helps him to not give into the Gear nature and lose his rationality, the headband also suppresses much of his power and also conceals the glowing red brand on his forehead the mark that would expose him as a Gear.
In terms of gameplay, Sol's moveset is similar to that of an aggressive Shoto-clone. His Gunflame projectile slowly covers a tall space in front of him to cover his approach or punish an extended poke, and his invincible Volcanic Viper uppercut is a strong reversal which he can threaten during wakeup or gaps in the opponent's blockstring. Many of Sol's normal moves lend themselves well to countering the opponent's pokes: Sol's sweep is extremely low profile, and can force many moves to whiff. His 6P renders much of his upper body invincible, and is quite rewarding if it counter-hits. He also has elements of a Grappler, as his Wild Throw command grab can be used to open up an opponent too afraid of counterpokes to contest his approach. In the corner, Sol players can do his iconic "Dustloop" combo, where he can string together repetitions of his jumping Dust attack several times over the course of the combo.
Sol can also spend 50% meter to enter the Dragon Install state, which powers up his entire moveset. However, if this power-up expires before the round ends, Sol enters a lengthy recovery state, giving a savvy opponent enough time to land an Instant Kill attack.