An Innocent Puppy Meets a Two-Faced Cat(2015)

A mild-mannered gentleman, good at his job, and reliable; Naohito's been wearing his mask of "perfection" for so long it's become as easy as breathing. That is, until he meets HIM. At the behest of a childhood friend, Naohito grudgingly attends a singles party for twenty- and thirty-somethings. With zero interest in mingling with the crowd, Naohito sits off on his own - and meets a much younger man, Tohru Akasaka, who clings to him like a puppy. Naohito's barely able to cope with Tohru's youthful energy, and they go their separate ways... but in a twist of fate, end up meeting again. Naohito's life is flipped upside down by his companion. Will he give in to this young man and let his mask drop!?

(Source: Renta!)

Note: Includes one extra chapter.