Sadamitsu Usui appears in the 2011 anime film "Legend of the Millennium Dragon" as a warrior serving Raiko Minamoto during the Heian era storyline. Identified as a sword fighter, he wears a headband and possesses dark skin in his character design. He operates alongside Raiko, Tsuna Watanabe, and Kinta as part of the group known as the Four Guardian Kings.

During the temple conflict where human forces battle the Oni, Sadamitsu engages in combat and defensive actions to protect the temple grounds. He later searches with Tsuna and Kinta for the protagonist Jun Tendo after Jun escapes with the Oni Mizuha, ultimately reuniting Jun with Raiko.

The character draws loose inspiration from historical and legendary accounts of Usui Sadamitsu, a mid-Heian period warrior who served Minamoto no Raikō and appeared in otogizōshi tales and Konjaku Monogatari as one of Raikō's Four Guardian Kings. Within the film, Sadamitsu's role remains secondary, confined to combat support and group dynamics without expanded backstory or character development. His portrayal is limited to this single film.