Long Yue is the turtle familiar assigned to Beast Knight Yukimachi Tsukishiro. He maintains an unwaveringly laid-back demeanor in all interactions. His core function involves supporting Yukimachi Tsukishiro's role as the Tortoise Knight, especially in combat where their combined abilities are essential.

He and Yukimachi coordinate closely with Beast Knight Subaru Hoshikawa (the Rooster Knight) and her familiar, Lee Soleil. To merge their psychic powers, they execute synchronized attacks requiring physical contact by holding hands. This collaboration activates their two signature techniques: the "Invincible Shield," which creates a vast, impenetrable defensive barrier that can reflect assaults, and the "Strongest Spear," a focused offensive blast intended to destroy threats.