How I Found Out My Girlfriend Was Cheating On Me and Somehow Created a Yandere Harem

Source: https://ncode.syosetu.com/n4346ie/

synopsis

Sagami Kanata, a high school sophomore, witnesses his girlfriend, Manaka Saki, cheating on him at the end of a part-time job he was suddenly asked to do.
They have been dating for two years. As their anniversary was coming up, he was hurt and on his way home in disappointment, he met a junior colleague, Yuzuriha Manaka, who had an experience of being cheated on, and eventually decided to break up with her girlfriend.
And in the days of gradual recovery after finally breaking up with the girlfriend who cheated on him, Kanata has an unexpected encounter.

Meanwhile, unbeknownst to him, various junior students and childhood friends with heavy affections are playing a dark role in his life. ……