Shougai Kimi ni wa Kanawanai

In elementary school, Shin was the boy who took the top spot from Aya, who used to be always in first place. When he joins Aya's company, Aya once again finds herself stuck in second-place hell... Aya is determined to be the best, so she challenges him to a contest at any cost. Shin, who acts surprised by her challenge, accepts on the condition that "the loser will do what the winner says," but he has an agenda of his own...?
(Source: Petit Comics, translated)