Leviathan acts like a refined, honorable seegentleman who is fiercely loyal to his host, Squigly. Speaks with a sophisticated English accent and aristocratic demeanor, balancing polite elegance with a dry, sharp wit toward enemies. Deeply caring, protective, and constantly looking out for Squigly's well-being, often expressing genuine guilt or concern if she is placed in danger. Despite his gentle nature, he harbors a strong sense of pride and can tap into a feral, dragon-like intensity in the heat of battle to defend those he loves.

Valentine is a combat medic and double agent in Skullgirls, defined by her dark ninja-nurse aesthetic, high-mobility rushdown playstyle, and pragmatic nature. Originally named Valerie, she was second-in-command of "The Last Hope," an elite squad of Anti-Skullgirl Lab operatives. After her entire team was wiped out by the Skullgirl, she surrendered out of self-preservation and agreed to serve the current Skullgirl, Marie. ​In the story, Valentine works closely alongside the shapeshifting monster Double as a duo of "partners in crime" who manipulate events from the shadows, though Valentine secretly acts as a double agent to gather research and eventually destroy the Skull Heart. Visually, she sports a dark nurse uniform, surgical mask, and long blue hair covering an eyepatch, wielding scalpels, giant syringes

Double is a horrific, shapeshifting abomination in Skullgirls that usually hides under the guise of a serene, smiling nun named Sister Agatha. Created to serve the divine Trinity, this formless monster manipulates events behind the scenes to ensure the Skull Heart constantly finds new hosts to spread chaos, morphing into gruesome amalgams of other fighters during battle. ​While Double acts as a guide to the current Skullgirl, Marie, their connection is far from a genuine partnership. Double originally manipulated a young Marie into wishing on the Skull Heart, positioning herself as a mentor, but Marie's strong will allows her to resist the artifact's corruption to pursue her own vengeful crusade. Seeing Marie as an unpredictable tool, Double constantly plots behind her back to find a more obedient replacement. ​The "partners in crime" dynamic comes alive through the tense, reluctant trio formed by Double, Marie, and Valentine. After Double and Marie slaughtered Valentine's anti-Skullgirl operative squad, Last Hope, Valentine surrendered and pledged her allegiance to save her own skin. Serving as the Skullgirl's enforcers, Valentine operates in the shadows while plotting her own secret revenge

Marie Korbel is the tragic, maid-outfitted Skullgirl who serves as the primary antagonist of Skullgirls, wielding a massive vacuum cleaner and bone-based magic to seek vengeance against the Medici Mafia. To protect the Skull Heart and carry out her crusade, Marie relies on her forced allies, Valentine and Double. Valentine acts as her tactical chief operative out of self-preservation while secretly seeking a way to destroy the Skull Heart, whereas Double serves as a shapeshifting, monstrous guardian manipulating events to ensure the dark artifact's ultimate will is fulfilled

Filia Medici is a core protagonist from the fighting game Skullgirls, depicted as an amnesiac schoolgirl fused with a unruly hair-parasite named Samson. ​Visually, she wears a modified school uniform consisting of a white collared shirt, dark necktie, miniskirt, and thigh-high socks. Her defining physical trait is her thick, dark hair, which is actually Samson—a sentient creature with sharp teeth, large eyes on the back of her head, and the ability to shapeshift into tentacles, jaws, or giant drills during combat. ​Personality-wise, Filia is kindhearted, gentle, and somewhat naive. Despite her connection to the corrupt Medici mafia family, her memory loss leaves her mostly innocent to their crimes, relying heavily on Samson's protective, albeit brash, nature as they search for answers about her past. ​In gameplay, she operates as a fast-paced rushdown character. She uses Samson's morphing abilities to quickly close gaps, execute fast air-dashes, and overwhelm opponents at close range with relentless mix-ups and combo pressure

Samson is a sentient, parasitic symbiote in Skullgirls who is permanently fused to the scalp and hair of his host, Filia. Visually taking the form of long, dark, raven-black hair, Samson features glowing yellow eyes, a massive toothy grin with razor-sharp teeth, and a gaping mouth positioned at the back of Filia's head—a design inspired by the Japanese mythical futakuchi-onna demon. He acts as Filia's primary source of power, using his shapeshifting capabilities to morph her hair into devastating weapons like giant fists, sharp blades, drills, and ferocious beastly jaws to crush opponents. Personality-wise, Samson is brash, rowdy, and fiercely sarcastic, often acting as a comedic contrast to Filia's polite nature. Despite his morally ambiguous past and tough, street-smart demeanor.

Parasoul is the disciplined crown princess of the Canopy Kingdom who fights using a living umbrella that shoots explosive napalm tears alongside her military squad, the Black Egrets