Gremmy Thoumeaux is a Quincy and a member of the Wandenreich's elite fighting force, the Sternritter, holding the designation "V" for "The Visionary." His true essence is not the childlike human form he typically presents, but rather a disembodied brain preserved within a protective casing. The physical body he uses, featuring messy blond hair, youthful features, and striking red eyes, is itself a creation of his imagination, as is his modified Sternritter uniform which includes a white trench coat with a hood and high collar. Due to the immense danger posed by his abilities, he was kept sealed in a special prison by Yhwach before the invasion of Soul Society.

Gremmy's personality is a complex and contradictory mix of arrogance, cruelty, and profound loneliness. He holds the absolute belief that imagination is the greatest power in the world and considers himself the strongest of all Sternritter, a claim he feels needs no proof. This pride is matched by a casual disregard for life, including those he creates. He coldly erases his own construct, Guenael Lee, for proving too weak, and callously executes two incapacitated Soul Reaper captains. During battle, he displays a sadistic streak, such as transforming Yachiru Kusajishi's bones into cookies to maim her. Despite this cruelty, he can maintain a veneer of politeness, feeling it inappropriate not to create a suitable stage for a worthy opponent like Kenpachi Zaraki. For most of his existence, Gremmy never felt the desire to destroy anyone, as no one had ever shown aggression toward him. However, during his fight with Kenpachi, he discovers a genuine wish to obliterate his opponent and is filled with joy at the prospect. In his final moments, this arrogance gives way to a deep sense of loneliness at the thought of dying and entering a void where he can no longer imagine anything at all.

In the story of the Thousand-Year Blood War arc, Gremmy is unleashed by Yhwach during the second invasion of Soul Society and told to destroy as he pleases. His primary role is that of a major obstacle for the Soul Reapers, showcasing the terrifying upper echelons of the Wandenreich's power. He infiltrates a medical barrier undetected, where he kills Captains Rōjūrō Ōtoribashi and Kensei Muguruma, who were incapacitated and undergoing treatment. After disposing of his own creation, Guenael Lee, he attacks Lieutenant Yachiru Kusajishi before being confronted by Captain Kenpachi Zaraki. Their ensuing battle becomes Gremmy's defining moment in the series. He unleashes an escalating series of imaginative attacks, from conjuring lava and missiles to creating a clone of himself, which doubles his power. His ultimate attacks include summoning a meteor large enough to destroy the Seireitei and creating a pocket of the vacuum of space. However, he is ultimately defeated when he attempts to imagine himself with enough physical power to match Kenpachi. His body, a product of his own imagination, cannot contain this immense power and disintegrates, leaving only his true brain form, which then dies.

Key relationships for Gremmy are limited but significant. His primary relationship is with his opponent, Kenpachi Zaraki, who serves as the catalyst for him experiencing genuine desire for battle and destruction for the first time. He also has a connection to his own creation, Guenael Lee, whom he treats as a disposable tool. Light novels expand his background, revealing a brief, amicable interaction with fellow Sternritter Liltotto Lamperd, where he creates pastries for her. After his death, his inert brain is harvested by Tokinada Tsunayashiro to create the artificial hybrid being Hikone Ubuginu, whose fighting style later carries mannerisms reminiscent of Gremmy.

Gremmy's development is tragic and brief. He begins as an arrogant, untouchable force who believes his imagination is without peer. Over the course of his single battle, he evolves from a detached observer to an active participant who desperately wants to win. He experiences the thrill of a challenge and the frustration of being outmatched. His final realization that he lost because he could not imagine a body strong enough to contain the power needed to beat Kenpachi shows a fatal flaw in his own ability: his imagination is ultimately constrained by his own perception of reality.

Gremmy's defining ability is his Schrift, The Visionary, which allows him to turn any fantasy into reality. This power is virtually limitless in scope, enabling him to manipulate matter by creating lava, water, and changing terrain; alter biological properties, as seen when he turned bones into cookies; instantly heal any wound by imagining it healed; and create sentient life, including his own clones or beings like Guenael Lee. Creating clones doubles his imaginative potency, allowing for even greater feats like summoning the massive meteor. As a Quincy, he is also highly proficient in reishi manipulation, granting him enhanced speed and the ability to infiltrate barriers. The Visionary has critical limitations. Its effects can vanish if Gremmy loses concentration or diverts his attention elsewhere. Furthermore, if he imagines his own death or physical limitations, those can manifest, leading to his defeat. His battle with Kenpachi underscores that his power is linked to his own mental fortitude and imagination; he cannot make himself stronger than he can conceive of being.