- Name: Boomerang
- Japanese Name: ブーメラン (Būmerang)
- Age: Unknown (Over 1,000 years old)
- Height: Unknown
- Weight: Unknown
- Race: Metal Demon
- Birthplace: Hiades
- Weapon: Sabre Fang
Nicknamed 'Cannibal,' and 'Executioner' by his fellow demons, Boomerang is an outcast, preferring to stick to himself. His only companion is the Guardian of Desire, Luceid, who feeds off of his desire to fight. Mother chooses him to be the fourth Quarter Knight after Berserk's death, against the wishes of Zeikfried.
His constant companion Luceid is the physical manifestation of the unquenchable desire Boomerang feels to fight. It is hinted by Berserk that Boomerang had a hand in the destruction of the Demons in the first Demon war. He feels the need to test his abilities and power on three different occasions to the trio. The first of these times he uses Luceid to trick the trio at the Epitaph of the Sea Wind, and then reveals himself to be with the Demons. Cecilia is shocked that a guardian would side with evil, until Boomerang reveals that Luceid is neither good or bad, but sides with the team that would allow her to retain a physical form. Being the Guardian Lord of Desire, she can retain her physical form being around Boomerang, who is endlessly searching for battle to quench his desires. Challenging the trio, both Boomerang and Luceid fail, and hands over the Rune Drive to them.
Boomerang is seen again in the Dead Sanctuary, blocking the trio from reaching the Goddess, Lion, and Dragon idols (being the trapped incarnations of the Guardian Lords of Love, Courage, and Hope). He is defeated again but is satisfied that he was able to test his limits against the trio once more.
He is not seen again until Ka Dingel rises from the depths. Playing little part in the overall storyline, Boomerang merely is used to delay the trio. Following a third defeat at the hands of the trio, Boomerang laments on his unquenchable lust for battle. Following this, Zeikfried (being possessed by Mother) tells Boomerang that they're finished with him and summons Demons to kill him and the trio. Boomerang and Luceid let them go, commenting on how they are taking a trip to hell together. If you enter and exit Ka Dingel following this, his Saber Fang will lie there, and the descriptions reads "I'll be back..."
If you beat the Ancient Arena once, and have the Saber Fang, Boomerang and Luceid will battle you a fourth time, except they will have merged into Boomerang Flash. Luceid has become the Guardian Blade of Desire, and is used by Flash to attack. Following this defeat, he accepts death and feels content knowing he has experienced his ultimate desire of battle throughout his long life.