Geniusman, alias Tensaiman, proclaims himself a born genius, evidenced by extraordinary intellect and strategic prowess. His appearance features a purple suit displaying a large yellow "天" (ten) kanji and smaller black "才" (sai) kanji on the torso, forming "tensai" (genius), complemented by dazzling blue diamonds on his elbows and knees. His blond hair (chartreuse green in anime) flares dramatically on the left side; the right side bears a blue crescent moon with a star, storing potent light energy. He typically wears a flowing cloak—deep blue in manga, bluish off-white in anime—with a coral interior and golden snap closure. An early manga design briefly featured a white-and-purple suit with ringed planets before adopting the current look. His face exhibits effeminate traits: jewel-like, pupil-less emerald-green eyes (later anime added pupils), a pointed nose, and visible fangs.

Originally, Geniusman served as the ultramarine "thumb" ranger, Oya Yubi Gunjo, within Yotchan's (Yonaoshiman's) villain team. His defection from the Hero Committee stemmed from losing his elite-status diploma during a mission. Depressed, he joined Yotchan's group—Speedman, Slimeman, Powerman, Kotetsuman—who also felt wronged. His allegiance shifted when Luckyman returned his diploma, found and signed by the Original Luckyman. Mistakenly believing Luckyman stole it, Geniusman attacked using a hypnotic gold pendant and the Kurushii Machine torture device. Luckyman's luck caused the device to trap Geniusman instead. After Luckyman freed him (motivated by desire for the pendant) and returned the diploma, Geniusman realized Luckyman's innocence and Yotchan's manipulation. He defected, joining Luckyman's team to fight Yotchan and other threats.

In human form, Geniusman disguises himself as middle school student Tensai Umaretsuki, a name punning "umaretsuki tensai" (born genius). This form retains his superhero hairstyle, though the moon and star symbols become regular hair. He wears a school uniform with a green blazer featuring shiny blue or yellow diamonds on elbows and knees, mirroring his suit, and a tie bearing the "天" and "才" kanji. He attends school alongside Endeavorman (Doryoku Sugita) and his brothers, maintaining a fierce rivalry with Victoryman (Shori Isono) in both identities.

Geniusman's personality blends aristocratic calm with intellectual arrogance. He frequently strokes his hair while uttering "Hmph" and refers to himself in the third person as "this genius" (この天才). He often adopts a romantic, solitary pose, contemplating events with closed eyes and declaring his actions "understood only by geniuses." However, mention of his past triggers a volatile temper, escalating from threatening his "Kah" attack to feral rage. Despite his bravado, he acts as the tactical "brains" of the Hero Committee's team, devising unbeatable strategies against alien threats. His confrontations with Victoryman remain perpetually interrupted.

His powers center on genius-level intellect, enabling advanced deduction, mathematics, tactical planning, gadget construction, and combat skill. Key abilities include:
- **Light-Speed Punch**: An ultra-fast punch delivering massive force.
- **Kah Kōsen (カッ光線)**: A devastating beam fired from his head's moon and star, capable of atomizing targets. Variants include the anime's "Moonlight Special Kah" and the manga's "Pikah" (incorporating "pika" sparkle onomatopoeia). The beam destroys most targets and exposes disguised foes, though Victoryman once survived it. Geniusman can access this power even in human form.
Transformation into Geniusman requires Tensai Umaretsuki to articulate profound intelligence—a complex equation or theory—triggering a beam of light.