Hailing from Japan, Aldebaran—commonly called Al—arrived in the world of Re:ZERO before Subaru Natsuki. While his exact background remains hidden, the resemblance in their summonings hints at the Witch of Envy's space-time manipulation. Now, he serves as a knight for Priscilla Barielle. Linked to the Witch of Envy, his Authority creates localized time loops within a set area. This power preserves his memories through each reset, letting him replay confrontations and exploit opponents' predictability. Despite being physically outclassed by warriors such as Garfiel, his Authority provides tactical dominance: he anticipates attacks and lands precise counterstrikes. Additionally, he wields earth magic to reconstruct lost limbs or form projectiles, though prolonged combat severely drains his stamina. Central to his motivations is his relationship with Echidna, the Witch of Greed. She bestowed the name "Aldebaran"—meaning "Following Star"—to signify his role: defying the Witches' expectations while carrying their hopes. Though Echidna criticizes his failures, she extends conditional support, even offering her own body to fuel his resolve. She attributes his suffering to Subaru's inadequacy, declaring, "Natsuki Subaru failed you." Despite his hatred for both the name and Echidna, Aldebaran persists in dream-space encounters to further his undisclosed mission. He commands the shinobi Yae Tenzen, whom he traumatized into absolute submission using his Authority's "Aggressor" aspect. Yae serves him obsessively, driven only by the wish to witness his death after his mission concludes. Aldebaran displays hostility toward Subaru, reacting with visceral disgust to the mention of Rem—a reaction suggesting awareness of timeline alterations. In dialogues with Echidna, he vows relentless pursuit to "kill" an unnamed target, promising to persist "even beyond a billion" attempts.