Isshou Watashi no Joshu de Ite!

The relationship between detective and assistant is "love"!

Gokusukeku is tired of getting taken advantage of, due to his natural disposition to help others, so his new goal in life is to “be in charge” of any situation. However, when he transferred to a new high school, he reunites with three girls who worked together as detectives back in elementary school! Sukeku's heated battle for the position of being the detective (in charge) begins! However...the girls have feelings for him in a different way…!

(Source: Magapoke, translated)