Breast-flattening shirt

The breast-flattening shirt is a short sleeved, front-buttoned flannel shirt that bears a curse. Any woman that wears it will experience a significant flattening in the appearance and size of her breasts, to the point where she may be able to pass for a man. The effect is increased if men are more than a little oblivious to changes in a woman's hair style and clothing.

Sedine Kilgarriff originally owned the shirt until she gave it to Ellie Gardner to help Ellie improve her relationship with Hank Durum. As "Al", Ellie was able to be around Hank and participate in sports-related conversations and activities. The disguise was very effective on him and he only thought of her as a guy. It wasn't until "Al" started putting the moves on him in a Las Vegas hotel room and the wig and shirt came off, instantly transforming herself back into Ellie (complete with her normal bust), that Hank finally realized that she and Al had been the same person all long.

Background information
Bea R. revealed in a Q&A post on Webtoon that the most often-asked question she received was how a woman like Ellie could have such a radical change in that part of her appearance. Bea explained that when Sedine was created, she was intended to be a very flat woman (part of the "A-Team"). When first introduced in 2017, she was closer to bust size, but as time has passed, she has become more curvy. Bea then came up with "the curse of flatness" being attached to the shirt.

It should be noted that if the curse comes from the shirt, then it may have a cumulative effect or proximity may be a factor, given that Sedine was shown many times not wearing the shirt when she was at her smaller chest size.