From Comic-Walker:
The idol who transfers into the protagonist's school wants to be supported by him, even more than his girlfriend. Keita Azuma, who used to chase after idols, stopped supporting them after he got himself a cute girlfriend, the first in his entire life. One day, Keita's "oshi" (favorite) idol transferred into his classroom and he didn't know if it was good luck or bad luck. "Support me, not your girlfriend."
She made a strong push on Keita. Just when he thinks his glory days have come, his green spring turns gray, as he becomes captivated by this idol's allure and they fall into an immoral relationship.