Black Ranban rules the galactic empire and acts as the primary antagonist in the series' first season. Visually resembling Darth Vader, Ranban wears distinctive black armor and a helmet. Belonging to the same species as Mito, Ranban shares a unique biology where gender is determined during puberty. Imprisonment resulted from Ranban's unstable gender biology, which prevented ascension to the galactic throne. This instability fueled intense resentment against royal relative Mito, whose stable gender allowed rule. Seeking revenge, Ranban eliminated the royal family, leaving Mito as the sole survivor, and pursued Mito across the galaxy.

A pivotal duel between Mito and Ranban culminates in Ranban's helmet removal, revealing an identical face to Mito's robotic "Mail Suit" appearance. This confirms their shared biological ties, practically twins except for gender stability, and Ranban's use of the same powered armor technology. Ranban later kidnaps Mito's son Aoi to force surrender, attempting to remove a bracelet believed to suppress Aoi's power. However, the bracelet actually restrains it; removal unleashes Aoi's full abilities, undermining Ranban's scheme.

In the second season, Ranban undergoes a significant shift, joining forces with the protagonists to confront a new, larger threat: the goddess Hikari. This transition aligns Ranban with former enemies against this greater danger.