Usotsuki Paradox

Owada Wakame is innocent and inexperienced in relationships, and she sometimes fears she will end up all alone. She and her friend Ari notice a new employee at their company, who seems pretty attractive to both of them! Unfortunately, the man in question, Kanesue, has his own hangups about the opposite sex, based on some less than pleasant experiences he's been through. Both Kanesue and Wakame see each other as someone different than how they really are, and their assumptions keep getting in the way. Will Kanesue and Wakame be able to find what they need in love? (Source: MangaHelpers)

Did you kiss a girl who has a boyfriend?? Yokado, who was in charge of "a substitute for lonely times" for Seiyu, who is in a long-distance relationship. However, her flirtatious relationship eventually becomes serious, and Yokado becomes Seiyu's true lover.