Nijuu to Seijuu

In her third year in the workforce, Aya, who has just turned 20, has her first junior colleague! His name is Yamato. He is six years older than her at 26 years old. He has a refreshing and caring attitude, which made him popular with the department's other colleagues, but he likes to tease Aya a lot and bully her a little bit, even though she was supposed to be the one training him… When the smart and mature side of Yamato-san takes the lead, Aya gets all flustered. At this rate, she won't be able to work...! A secret office love affair with a slightly sadistic older junior…

(Source: Betsufure, translated)