Light, a young Maiasaura, is the biological son of a female Maiasaura who adopts an abandoned egg containing a Tyrannosaurus later named Heart. He hatches alongside Heart and is raised as his adoptive brother by their shared mother. She chooses exile from her herd to protect Heart after the herd leader threatens the carnivorous hatchling. Physically, Light has blue skin with yellow markings, contrasting sharply with Heart's appearance.

During their childhood, Light and Heart engage in sibling play-fighting, where Light consistently defeats Heart in contests like arm wrestling. This dynamic frustrates Heart, often reducing him to tears. Light occasionally makes insensitive remarks about Heart's differences, joking about his resemblance to carnivores from warning songs, which deeply wounds Heart. Their mother intervenes to correct Light, reinforcing their family bonds.

A pivotal confrontation occurs when the Tyrannosaurus Gonza attacks them, attempting to force Heart to embrace his carnivorous nature by targeting Light. Light instinctively shields Heart, shouting, "He's my brother!" Later, after Heart briefly succumbs to predatory instincts upon tasting meat and nearly attacks Light, Heart flees to protect the family, leaving Light with their mother.

Years later, Light rejoins the Maiasaura herd as an adult guardian. When Heart returns to warn of an impending volcanic eruption at Egg Mountain, Light reveals their mother lives separately. Heart criticizes Light for not searching for her, exposing lingering tension. Light aids the evacuation, rescuing Heart and their mother from a cliff fall. During the reunion, Light meets Heart's adopted Ankylosaurus son, Umasou, though their direct interactions remain minimal.