Umasou is a baby Ankylosaurus first introduced as a newly hatched egg discovered by the Tyrannosaurus Heart. Upon seeing him, Heart remarked "Umasou," meaning "you look delicious," which the hatchling misinterpreted as his name.

He formed an immediate and unwavering attachment to Heart, instinctively identifying him as his father despite their differing species. Umasou displayed an energetic, trusting, and affectionate nature. He once ventured out alone to gather berries to share a meal with Heart, demonstrating his caring disposition while inadvertently placing himself in danger.

His relationship with Heart evolved from one-sided dependence into a mutual father-son bond. Heart trained Umasou in survival techniques such as roaring and defensive maneuvers to protect himself from predators. A significant moment occurred when the Tyrannosaurus Gonza confronted him with the truth that Heart was not his biological father. Umasou affirmed that he was already aware but still considered Heart his true father, highlighting his emotional intelligence and the strength of their adoptive bond.

Physically, Umasou grew considerably throughout the story, eventually reaching a size where he could no longer fit inside Heart's mouth, a place where he was once kept safe during conflicts. The character appears in the original picture books, which have multiple adaptations. In one literary version, the ending differs significantly from the film adaptation; Umasou is ultimately reunited with his biological parents and leaves Heart to live with them. Another adaptation, a television series of short episodes, features a story arc where Heart interacts with five baby Ankylosaurus, though Umasou's specific role in that narrative is not detailed.