Mysterious Girlfriend X

Mysterious Girlfriend X is a 2006 manga series with a 2012 anime adaptation about a student named Akira Tsubaki who gets a new mysterious girlfriend named Mikoto Urabe who has many eccentricities. They form a unique relationship.

Episode 3
At 15:18, Mikoto Urabe takes a Shower.

Mysterious Girlfriend X OVA
After attending a festival with Mikoto Urabe, Akira has a flashback to when he was younger and he attended one with his elder sister Youko Tsubaki. After explaining that she is sweaty from walking around, Youko announces her intent to have a bath, and is seen taking off her Yukata, and wearing nothing underneath it. Akira remembered this after Mikoto explained to him that she wasn't either.

While the flashback ends before she actually gets in the bath, she is right outside the bathroom and a towel rack and tiles can be seen behind her.