Regulus

Regulus in High School DxD is the spirit of the Nemean Lion sealed within the Longinus Sacred Gear Regulus Nemea, wielded by Sairaorg Bael.

He appears as a massive golden lion (about 4-5 meters tall) with a distinctive jewel on his forehead, and he can also take a human form resembling a young boy with spiky dark orange hair, golden eyes, and facial markings.

Regulus is extremely loyal to Sairaorg (whom he calls "Sairaorg-sama" or "Master"), serves as his Pawn in the peerage, and acts as an independent avatar for the Sacred Gear. The gear itself manifests as a powerful golden battle axe capable of immense destructive force (like splitting the earth) and projectile resistance, while Regulus can manifest separately to fight alongside his master.

He first becomes prominent in the Rating Game arcs involving Sairaorg, especially in season 3 (High School DxD BorN) and season 4 (Hero), where his bond with Sairaorg and raw power get highlighted in intense battles.