Sin Kiske is a vivacious, big-hearted young man. Though often thought of as unintelligent and treated like a child, his honesty and optimism quickly changes people's minds about him.

To say Sin has had a rough life is a massive understatement. Born of a secret marriage between the human Ky Kiske, hero of the Crusades and all around a beloved figure, and the Gear Dizzy, who had a bounty put on her head merely for existing, Sin's existence was quickly discovered by the United Nations. He and his mother were then essentially held ransom in order to coerce Ky into becoming a puppet king of the newly-founded United Kingdoms of Illyria. In order to protect him from further harm, Ky placed the now toddler-sized Sin in the care of his friendly rival, Sol Badguy. While this arrangement did indeed keep Sin safe, he unfortunately was so young, and his caretaker so dismissive of his father, that he utterly despised Ky for a time out of feeling as if he was abandoned. This was eventually resolved, however, after Sin worked alongside Sol, Ky, and the mysterious fox youkai Izuna to stop Valentine from fulfilling her creator's plans to annihilate humanity. By the events of Guilty Gear Strive, he also managed to change Ramlethal Valentine's heart so thoroughly that she went from merely being Valentine's successor who was fast-tracking another apocalypse, to being a well-respected and compassionate commander in the Illyrian armed forces.

Despite his youth and simple nature, Sin is a very competent fighter, having both inherited his father's lightning magic and endured arduous training while traveling with Sol. He also sports a modified Illyrian battle standard, rolled up and wielded like a polearm. Though Sin's lightning magic is powerful, he does still have some difficulty controlling it, so his eyepatch is actually a seal preventing him from losing control and causing harm to himself and others. Underneath it is a bright red eye, marking his nature as a half-Gear. Overuse of his lightning also expends a great deal of energy, so Sin is often very hungry, snacking on anything vaguely edible in the middle of a fight in order to regain his stamina. Food means a great deal to him in general, though never say the words "sugar water" around his father...