Isshou Watashi no Joshu de Ite!

"The relationship between detective and assistant is not unlike romance!" Because of his natural tendency to assist others (a "helper type" personality that makes him constantly want to lend a hand), Tasuku Mitomo has always lived in the shadows. To get out of this rut, he has made it his utmost priority to "become number one." However, at the high school he transfers to, he encounters three detective girls, who turn out to be his rivals from elementary school with whom he had formed a detective club! Thus begins Tasuku's fierce battle to support the top detectives! However... these girls seem to harbor passionate feelings of a different kind...!?

(Source: Kodansha, translated)