Kimura-kun wa Otoko Tomodachi

Miho and Tetsuo are a happy couple no matter how you look at it. Miho and Kimura have been friends since school days. Miho uses the Osaka dialect only when she talks to Kimura, and she looks really happy. Kimura knows a lot of Miho that Tetsuo doesn't know. Kimura is a narcissist, a flirt, and a talented novel editor, but he's about to get married with a younger girlfriend. So, even though Kimura passed under Tetsuo's radar, Kimura broke up with his girlfriend and quickly approached Miho!?
"The thoughts of the three of us began to become entangled..."

(Source: Kodansha, translated)