Title: Sayonara Game
This is the 1st entry in a shared world created by Yuu Minaduki. Each entry or series is self-contained but it's recommended to be read in order for the best experience.
Arimura Ikuo adores his younger sister and wants what's best for her. As the college exams near, he's determined to ensure she has the best chance at getting into the college of her dreams. Unfortunately, his plans go awry when his high school kohai, who has been tutoring his sister, has to bow out. Thankfully, the friend has another mutual classmate he offers to tutor her in his stead. He is Yosuke Ito, another of Arimura's kohai who also played on their high school baseball team. While Arimura fondly recalls much of his high school baseball career, he doesn't remember Yosuke, but that doesn't matter all that much. As long as Yosuke can help his sister study, he's happy with almost anyone.
Yosuke is a bit standoffish and quiet but is otherwise acceptable. With his sister squared away, Arimura can focus on his own issue: his work crush. Arimura is popular and successful at his job, but he's not all that confident when it comes to romance. So, when he finds out the object of his affection is being pursued by the chief at work, he encourages her to be with his rival rather than try and fight for her. Though it was his decision, he's heartbroken and bemoans his poor luck in finding love. As it turns out, Yosuke happens to be at his house, and he has been most days as he helps Arimura's sister prep for exams. But Arimura also notices Yosuke's attention is often on Arimura himself. And, in turn, Arimura finds himself watching Yosuke. The tension is high, and Arimura soon realizes this is another crush.
Unfortunately, his rival in love this time is his very own sister. His sister has been doing her best to seduce Yosuke, and though Arimura isn't willing to give his sister over to someone, he also finds himself not wanting to give Yosuke up to anyone, either. Can Arimura stop his self-sabotaging ways, or will he say ‘sayonara' to another potential partner?