Gremmy Thoumeaux is a Sternritter of the Wandenreich, serving under Yhwach with the designation "V" for "The Visionary." His true essence is a disembodied brain preserved within a protective casing, though he typically manifests a childlike human form featuring messy blond hair, youthful features, and striking red eyes. He wears a modified Sternritter uniform: a white trench coat complete with a hood and high collar. His personality is a complex mix of arrogance, cruelty, and profound loneliness. Gremmy declares imagination the ultimate power and considers himself the strongest Sternritter, displaying open disregard for allies and enemies. He erases creations like Guenael Lee for perceived weakness and casually executes incapacitated captains. In battle, he exhibits sadism, such as transforming Yachiru Kusajishi's bones into cookies to maximize suffering. Despite this, he maintains a veneer of politeness and experiences fleeting joy when facing worthy opponents like Kenpachi Zaraki. In his final moments, he expresses loneliness at the thought of ceasing to imagine. During the Thousand-Year Blood War arc, Gremmy infiltrates a Soul Society medical barrier, kills captains Rōjūrō Ōtoribashi and Kensei Muguruma, and maims Yachiru. He confronts Kenpachi Zaraki, conjuring elaborate constructs including lava fields, water prisons, and firearms. When these fail, he clones himself to amplify his power and summons a city-destroying meteor. After Kenpachi destroys the meteor, Gremmy traps him in a space vacuum but exhausts himself trying to match Kenpachi's physical strength. His body disintegrates when he unconsciously imagines himself incapable of sustaining such power. Dying, he reveals his brain form and laments entering a realm devoid of imagination. Light novels expand Gremmy's background. He used "The Visionary" to create the sentient being Shaz Domino, though Shaz escaped control by replacing his imaginary body with real Reishi. Gremmy shares a brief, amicable interaction with fellow Sternritter Liltotto Lamperd, creating pastries to satisfy her hunger while expressing cynicism about the Wandenreich's inherent selfishness. After his death, his inert brain is harvested by Tokinada Tsunayashiro to form the artificial hybrid Hikone Ubuginu. Observers later note Hikone's fighting style carries mannerisms reminiscent of Gremmy. Gremmy's Schrift, "The Visionary," allows him to manifest imagined concepts as reality. This includes manipulating matter (generating lava, altering terrain), transforming biological properties (converting bones to cookies), instantaneous healing, and creating sentient life like Guenael Lee or self-clones. Clones double his imaginative potency but require active focus to maintain invulnerability. His power carries inherent limitations: creations vanish if he loses concentration, and imagining his own death or physical limitations can manifest them. His defeat against Kenpachi underscores that his abilities remain constrained by his perception of reality. Gremmy possesses high-speed combat reflexes, effortlessly dodging lieutenant-level attacks, and wields immense spiritual energy feared by other Sternritter. His reishi manipulation excels in environments rich in spiritual particles, such as Soul Society. The novels suggest his power may originate from an aspect of the Soul King, akin to Gerard Valkyrie or Pernida Parnkgjas.