Doukyuusei Seikatsu

Mizuno and Mitazon were middle school classmates with polar opposite personalities. One day, years later, Mitazon calls Mizuno up to help take care of some stray kittens. Mizuno agrees to stay in his house to look after the kittens for a short while. This is living together with a classmate!
(Source: Shinshokan, translated)
Note: Contains the extra "Doukyuusei +1" (同級生+1).