44-sai no Kanojo

The protagonist is Ouji, a 44-year-old unmarried man who lives at his own pace. One day, Ouji barges into the apartment next door to try to stop a fight between the couple living next door, and meets Kiri, a woman his age who has also barged in... With the arrival of Aoi, a 32-year-old mentally unstable girl, Ouji's life reaches a turning point.

(Source: Natalie, translated)