Born as Frederick Bulsara in the 20th century, he was originally a scientist who specialized in Magic Physics since the interfacing of an evil entity on all normal technology lead to its banning. "That Man" was his closest friend, and he fell in love with Aria Hale during his college years; it was fate the three friends where chosen by their professor to work as the core team of the Ecosystem Evolution Project. A year later, "Gear cells" were successfully stabilized, and a year after that, the project caught the attention of the military. That Man became the head of the project, and forced it to become public and of non-military nature, renaming it to the "Gear Project". Just then, Aria revealed she was on death's door due to an incurable disease; Frederick begged her to enter cryosleep until she could be cured, but she refused. One day, he proposed to Aria; she offered to leave him something to remember her by, and Frederick picked her hat. When the military infiltrated the project, That Man managed to convince Aria to what Frederick proposed, but forced Sol into sleep as well, and inserted one of the two "seeds" that originated the Gear cells into him without his consent, restructuring him into the zeroth Gear, and faking his death. That Man and Aria went missing, and the project was frozen. When Frederick woke up, That Man denied ever being his friend, and disappeared along with Aria's body.
Designing a headband to keep his mutating self at bay, at 15 years since the start of the Gear Project, Frederick became a bounty hunter with a terrifying presence in underground organizations in order to find That Man and enact his revenge, becoming known as "Badguy". 15 years later, the US restarts the Gear Project for military use, successfully creating the first "Gear" lifeform, ineffective in combat, and difficult to control. That Man joined the project under a different identity, and in 20 years, the first combat-ready Gear was created, and mass-production began, allowing the US to practically control most of the world through the distribution to its allies. Badguy thus made it his mission to destroy every Gear, all the sinful spawn that came from him and his research. This picked the interest of Slayer, who hunted down Badguy and was impressed by his passion "like the sun", bestowing upon him the name of "Sol".
In 2073, That Man used all he had developed to implant the other "seed" into Aria and create Justice, the command-type Gear that could control all others. By sacrificing and transforming his other friend, he hoped to hijack the Gear Project and stop them from ever partaking in any conflict again, but during a test one year later, but it interfaced with the evil entity and started going haywire. When That Man realized the gateway it was employing was the japanese people, he forced Justice against her will to vaporize almost the entirety of Japan, and event dubbed the "Black Sunrise". The manifestation stopped, but the event broke Justice (Aria)'s mind, leading her believe the reason she did that was to rebel and declare war on humanity.
In response, Sol created the "Outrage", a weapon so powerful he divided it into 8 smaller weapons due to its uncontrollable power, and continued his private war against Gears as the Anti-Gear Crusades began. During the next ~30 years, he saved a young Kliff Undersn, who once grown and as the leader of the Sacred Order of Holy Knights, spared no means to eventually recruit him, and was placed under Ky Kiske's command. Ky challenged Sol to a duel; Ky fought Sol fairly—unaware of his nature—and lost. Sol interpreted it as Ky not taking him "seriously", and Sol's personality and usual disregard for Ky's orders led to a growing tension. Sometime after the Battle of Rome, Ky caught Sol during the act of stealing the Fireseal, but Sol defeated him once again and got away. Around 100 years after Justice's creation, after Kliff and Ky pushed her to retreat, Sol faced Justice for the second time. Completely shocked at not being able to control him, Sol used his chance to defeat her, then disappeared as the Holy Knights moved in and sealed Justice.
5 years later, one of his bounty marks tells Sol about the Second Holy Order Selection Tournament before Sol ends his life. Sol takes it upon himself to unravel the true intentions behind it, and becomes its champion, only to learn that Testament used the blood shed to free Justice. Sol manages to fatally wound Justice. When he reveals himself to be immune to her control because he's the prototype Gear, Justice finally remembers Frederick, and wishes that "the three of us" could talk one last time, and Sol—unaware of her identity—swears to kill That Man.
Less than a year after the tournament, Sol hears news of a still-functioning Gear with a high bounty and sets out to investigate. Sol finds Dizzy and overcomes Testament, her guardian. He's forced to resort to activating Dragon Install to match her power, and has a flashback of one of his encounters with Justice, instinctively guarding against Dizzy's own Gamma Ray. Instead of killing her, he says he has made a mistake in thinking she or any Gears had no will to live, and leaves.
Weeks later, I-No starts wreaking havoc to get Sol's attention, laying a trap to kill him before he gets in That Man's way. Alerted by Axl Low, Sol tracks her down, however Raven intervenes and orders her to stand down, and That Man is forced to punish I-No. Later, Sol knocks Dizzy out of Necro's possession, brought about by I-No's actions, leaving just before Dizzy wakes up. Having stopped I-No's rampage, Sol decides to investigate and get rid of the P.W.A.B. and crosses paths with Ky. He acquiesces to continue searching for I-No instead, crossing paths with Crow and his Justice robo-copy. I-No, through usage of a time warp, forces Sol to battle his past self, and then she slyly kills his Order-Sol. He is returned to the present, and Ky finds him to settle their score once and for all. Sol wins again, and Ky decides he will no longer pursue Sol but makes Sol promise they will meet again before going their separate ways.
Years later, close to New Year's Eve, Dizzy—accompanied by her son, Sin Kiske—asked Sol to help Ky out, who had reached a breaking point due to his appointment as puppet king of the newly-formed United Kingdoms of Illyria. Sol found a deteriorated Ky praying at the church and provoked him. The fight dragged on and Ky was losing as usual, but light returned to his eyes. They fought until a frightened Sin grabbed Sol's leg and demanded him to stop. Ky then asked Sol to take care of Sin, and Sol accepted.
In 2186, at the Village of Witches, Sol and Sin find a bounty poster of Sol, which is Ky's call for help, but they are soon attacked by the Vizuel, whom they are able to repel thanks to Izuna. At the capital they meet Valentine, and Sol is shocked by her resemblance to Aria. Within the castle, the trio find the guards defeated and Ky caught in a seal. Sol and Izuna go retrieve Dr. Paradigm from Ganymede. He dispels Ky's seal. Sol immediately asks about Dizzy, but Sin storms out when Ky shows them Dizzy's sealed state. Sol and Izuna fight a brainwashed Sin, afterwards following Valentine. Ky leads his men into battle so that Sol's group can reach her, and Sol punches Sin to stay with Ky. Sol, Izuna and Paradigm find the gate to the Backyard opened by That Man, whom enchants the three so that they can withstand the Backyard's pressure after ignoring Sol's attacks. Izuna, Paradigm and the late-arriving Sin hold back Valentine, and once Sol breaks the key, he Valentine's final form while the others escape. As Sol resigns himself to remain trapped, That Man reappears, warning this isn't the end of the "merciless apocalypse" and urges him to live on. Sol inquires if Valentine was really Aria, but That Man says they already killed that girl. Sol vows to kill That Man, who will wait for him.
In preparation, Sol decides to fuse his Fireseal with the Flahing Fang into the "Junkyard Dog". Before tracking down the other pieces, theyreceive a message from Ky, as Ramlethal Valentine has declared war on the world. A week later, they defeat Ramlethal, but the Cradle activates over Babylon, and she moves in to kill Sol by self-destructing. However, he survives thanks to Ky releasing Elphelt Valentine, Ramlethal's sister who warned them and he had imprisoned, who renders her sister powerless. Sol is distrustful and admonishes Sin for his attempts at befriending the Valentines, but Sin rejects Sol's lonely worldview, and Sol second-guesses himself. A days later, en route to Zepp per Leo Whitefang's suggestion after a cradle apparition, Ky privately asks Sol if he doesn't care about what will happen once he has taken his revenge, adding that he, Dizzy and Sin will wait for Sol to "come home". The duo meets with Gabriel and Zato-ONE, who reveals the Conclave wants to resurrect Justice. After Paradigm and That Man's allied ambush on the Cradle fails, and the latter is imprisoned away by Bedman, Ramlethal finally talks: the Conclave will use St. Elmo's Fire and Illyria Castle to awaken Justice. During the operation, Sol removes Chronus from inside of Justice. In the aftermath of the Conclave's defeat, Elphelt, forced by her programming, reveals that everything went according to her mother's design before she and Justice are abducted. Tired of losing people, Sol swears to save Elphelt. Days later Raven asks Sol's help—accompanied by Sin and Ramlethal—in catching Jack-O' Valentine. On NOV 17, they catch her, and Sol finally learns that Aria was Justice all along, but Raven and Jack-O' leave. On the way back to the capitol, Sol is almost killed by a massive Gear. After Sol returns to the capital, Ky and Sol reflect on their past rivalry (as Ky is busy filling out papers), with the latter surprised at Ky's indifference towards his past as Frederick, as Ky says that Sol's future matters much more, acknowledging Sol as a friend and offering him whiskey hidden under his desk he never bothered to drink. Later and somewhat drunk, Sol lectures Ramlethal about "what true happiness should be to oneself", then takes her and Sin to Danny Missile. After a meeting to discuss the Information Flares with Zappa and Faust, Ky, Sol, Leo and Paradigm meet with That Man and Jack-O' Valentine, and learn about the Absolute World, That Man's motives, and Ariels being the Universal Will. Sol lays bare his guilt over mankind's state and killing Aria, only to be shocked by the revelation that Jack-O' merging with Justice will reconstitute Aria.As per their plans, Ky and Leo confront Ariels at the celebration of victory, during which she announces her identity to the world before retreating; Ky obtains the XYZ from her, but she erases her signal. Zappa and Randy manage to locate her, however, and leaving the capitol's defense to Dizzy and their allies, Ky departs alongside Sol, Sin and Jack-O' to the Celestial Altar aboard Johnny's Gullinkambi Dark. During the journey, Sin brings up if Dizzy is related to Sol; Ky is horrified as he realizes that Sol may be his father-in-law. Against all odds, they are victorious: Ariels is defeated by Sol, Ramlethal turns up alive with Raven to help, Elphelt is freed, and despite Daryl's attempt to destroy Justice (and Sol and Jack-O' along with her), Jack-O' and Justice merge thanks to Axl Low and a weakened Ariels.
After the battle, Ky and Dizzy invite Sin, Elphelt and Ramlethal to rest at their estate, although Ky isn't sure how long they all can stay as he intends to make his marriage public, but Elphelt brings news of Dizzy being publicly praised for helping defend the capitol. Yet, Ky ends up horrified as they discuss Sol's place in the family. At that moment, Sol arrives with the intent to fight. Ky and Sol take a walk, with the latter asking if their score is truly settled. Ky says it is, but Sol claims Ky was never "serious" in their fights, having never unleashed his killing prowess used against the Gears during the Crusades which made Sol fear for his own life. Sol however, then calls Dizzy a monster to goad Ky; angered, Ky fights dirty and wins easily, ending with Sol lying down on the ground. Sol then asked about the tactics the Holy Knights used against the Gears during the Crusades, and upon hearing Sol say that he wanted to learn how humans can face monsters back then, Ky then pieces together the purpose of their scuffle: Sol is learning how prepare for his final showdown with That Man. Ky upon understanding Sol's conundrum, decides to help him with his issue.
That Man–Asuka R. Kreutz–in a shocking move, turns himself in, in exchange for a chance to talk at the G4 Summit. Three weeks later, Sol and Jack-O' (still not remembering being Aria) are working together as bounty hunters, when Ky summons Sol back to the castle. Ky tells Sol about the U.S. request that he be at the G4 Summit and tries to hire him to catch I-No, but Sol decides to go on vacation with Jack-O' to Iseo first. However, after I-No lets herself be captured and an encounter with Happy Chaos, he returns to help Ky interrogate. Sol decides to take up the G4 offer and travels to the White House while Jack-O' interrogates I-No. The summit starts, but H.Chaos brainwashes the entire security staff and takes everyone hostage. Sol rescues Vernon and they contact Gabriel. H.Chaos then activates the White House's Tír na nÓg aircraft mode, making escape impossible. Sol and Vernon reach Asuka's room, and Asuka tricks H.Chaos into being ejected inside it. The crisis seemingly averted, Asuka says he will use Tír na nÓg to travel to the moon to keep the Tome safe, but will settle his score with Sol beforehand. He then gives Sol a choice: to let him correct his mistake (planting the Flame of Corruption in him), or kill him. He chooses to let Asuka remove the seed, restoring his 'humanity'. Unbeknownst to them, a disguised H.Chaos was still onboard. He gets a hold of the Tome, fused to Asuka, and restores I-No to godhood at his own expense. Sol attacks her with the Outrage, but is knocked unconscious, and she decides to grant humanity the powers she has, likely destroying the current world. Ky leaves to stop her, followed by Jack-O', who takes one last look at Sol. After waking up, Sol asks Vernon to bring his zeal stash, and then stops Jack-O' from sacrificing herself to save the world; Sol tells Jack-O' it doesn't matter if Aria doesn't return, but that he won't let Jack-O' go, even if it means the end of the world, for Jack-O' is Aria's "hat". While Ky and Axl distract I-No, Sol hits her weak point—thanks to Nagoriyuki—with his zeal pod-powered Outrage, seemingly killing her, although the Outrage takes its toll on his body. After a state funeral is arranged for "Sol Badguy" the hero, Frederick moves in with Jack-O' to Iseo. Having opened a junk garage, he starts working on a rocket to travel to visit Asuka.
