Ratio is another Hamatora PI with a doctorate. He is a doctor who helps Birthday out on handyman business. He is also a Hamatora PI, and Birthday's partner and childhood friend. His Holder ability is fastidiousness which allows him to see the structure of a person's body and use the information to determine their weak points. He wears an eye-patch over his right eye which when removed activates his Holder ability. After discerning an enemy's weak points, he then uses brass knuckles to melee them into submission.