I thought it meant leaving the energy drink next to him while he was working, but it doesn't work. Did I do something wrong? I'm in this situation now.
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Follow her to the bathroom. You can have sex with her while she's showering.
After re-entering the game, she may reappear in the bathroom.
To make her your assistant, place her outside on the first floor and have sex with her multiple times until the dialogue box appears. You can then recruit Kasumi.
Also, the Ghost Doll can be used on Kasumi, Momoko, Fuyutsuki Hinata, and Matsuzaka Aoi. If you're unsure, try different scenarios.
It must be turned on manually. In contract, press esc on the keyboard/open journal > Side activities > special items > turn on the doll.
Attention! don't let Ryoko know about the item. If she finds out, the item will be confiscated.