Heart begins as an abandoned Tyrannosaurus egg discovered near a river by a female Maiasaura, who adopts and raises him alongside her biological Maiasaura son, Light. Believing himself herbivorous, Heart consumes berries but cannot digest leaves or acorns like his family, fueling identity insecurities. A childhood encounter with the Tyrannosaurus Gonza forces Heart to confront his carnivorous nature; Gonza reveals Heart's true species and provokes an attack where Heart bites off part of Gonza's tail. Tasting meat awakens Heart's predatory instincts, and fearing he might harm his adoptive mother and brother, he exiles himself.

Alone in the wilderness, Heart hones his hunting skills and physical prowess, evolving into a fierce, survival-driven predator while retaining underlying kindness. Years later, he discovers an Ankylosaurus egg that hatches before him. Intending to eat the hatchling, Heart remarks "you look delicious" ("Omae umasoudana"), leading the newborn to name himself "Umasou" and imprint on Heart as his father. Despite initial plans to consume Umasou, Heart grows attached and becomes his protective guardian, training him against predators using methods reminiscent of his Maiasaura mother. Heart conceals his carnivory and orchestrates a race to trick Umasou into leaving, fearing abandonment. When Umasou faces Gonza's group, Heart rescues him, fully accepting fatherhood, an act that earns banishment by Tyrannosaurus leader Baku.

Learning of imminent volcanic activity at his birthplace, Egg Mountain, Heart returns to warn his adoptive family. En route, he confronts and defeats Gonza again. Reunited with his Maiasaura mother, who now has new offspring, Heart protects them during the eruption. He battles Baku, who recognizes Heart as his biological son and spares him, acknowledging the Maiasaura's role in raising Heart. Heart departs with Umasou, reaffirming their bond while maintaining ties to his adoptive family.

Physically, Heart is a dark teal Tyrannosaurus with thin red streaks across his body, blue eyes, and yellow sclera. His personality merges combat aggression with deep loyalty and family compassion, alongside persistent insecurities from his hybrid upbringing. He demonstrates strategic intelligence through tactics like exploiting terrain to exhaust opponents or feigning threats to manipulate herbivore herds.