Mathieu, known as Hiro Randou in the original Japanese version, is the eldest son of the Randou family in the French adaptation, which relocates the setting from Japan to Canada. As the family's firstborn, he holds significant responsibility, often assuming leadership in his father's absence. His combat expertise centers on precise knife throwing, which he employs with high proficiency, though he supplements this with firearms when tactically necessary.

Mathieu reliably pilots the Acrobunch mecha during battles against the Goblin Society forces, simultaneously handling tactical decisions during engagements. In one documented skirmish, he unexpectedly displayed chivalry by freeing the Goblin general Cera from a trapped vehicle—an act she misinterpreted as an insult.

His backstory includes the loss of his mother during his youngest brother Jun's birth, shaping family dynamics. Mathieu exhibits protective instincts toward his siblings, particularly evident when the Goblin Society destroyed their marine farm and killed his sister Reika's pet dolphin in their initial attack.