Toilet-bound Hanako-kun, Vol. 5 by AidaIro
Nene Yashiro serves as the assistant to the School Mystery Hanako-kun. When she is invited by enigmatic playboy Natsuhiko to a mysterious tea party, she meets Hanako-kun's girlfriend and his little brother...!? On top of that, she learns that one of the Seven Mysteries is mixed in with her classmates...