"Ano Toki Tasukete Itadaita Monster Musume desu." Isekai Ossan Kyoushi Totsuzen no Moteki ni Konwaku suru

Story:

Some 20 years have passed since the protagonist was summoned to another world and retired from the army. He is still single in his 40s and lives as a low-ranking professor, until one day, an elite 'monster girl' transfers into his class.