Karaoke de Utau Onee-san to Issho

Sano Tsukasa, a first-year university student who works part-time at a karaoke shop. One day, he encounters a female customer who used to be a children's TV singer. She pleads with him, saying, "If you don't mind, could you listen to just one song?" and "Honestly, I want to sing in front of children again, just like I used to..." Though puzzled, Tsukasa agrees. The woman turns out to be an incredible singer, but despite performing children's songs, for some reason, her singing is irresistibly sexy...

(Source: Kadokawa, translated)