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

Two decades have passed since the protagonist was summoned to another world, where he served in the military before eventually retiring from active duty. Now a man in his forties who has never married, he leads a quiet, unassuming life working as a low-ranking professor at an academy. Having left behind the battles of his youth, he expects nothing more than a peaceful and routine existence, content with his modest role in shaping the next generation within this unfamiliar realm. However, his tranquil daily life is abruptly upended when a new student transfers into his class. Far from an ordinary enrollee, she is an elite monster girl of exceptional standing. Her sudden arrival draws immediate attention, but her presence soon brings something far more unexpected for the aging educator: a wave of intense romantic interest directed squarely at him. Completely unaccustomed to such affection after years of solitary living, the middle-aged teacher finds himself utterly bewildered by this sudden and overwhelming popularity, forcing him to navigate a complicated new chapter in his otherworldly life.