She might be embarrassed. It's a rather private and unexpected situation.
The girl herself would likely be very embarrassed. She might turn red in the face and try to act like it didn't happen. Some classmates might giggle or laugh, especially the younger students. Others might be more understanding and not react at all, or they might even try to comfort the girl if they see how embarrassed she is.
Some readers might be shocked if they are not expecting such explicit content. Others might find it titillating if they are reading an adult - oriented story. However, many would also be critical if it seems out of place or just added for shock value without any real narrative purpose.
She can cross her arms over her chest to cover up.
She might be very shocked at first. It's an unexpected and rather private item to come across.
Perhaps she's just at home and wants to be as free and comfortable as possible. Bras can sometimes be restrictive and uncomfortable, especially if she has been wearing one all day. It could also be that she's in a private enough setting where she doesn't need to worry about the traditional norms of always wearing a bra.
One possible theme could be self - discovery. A character realizing that getting her period is a sign of growing up and starting to understand her own body better.
One possible reaction could be shock from society's more traditional viewpoints. In many cultures, this goes against the typical gender - role norms. However, in more progressive and open - minded communities, it might be seen as an exploration of sexual freedom and equality within a relationship.
She might be laughing uncontrollably while begging for the tickling to stop. It's a natural reaction as tickling can be both pleasant and overwhelming at the same time.
She could be flattered. In some fanfics, the girl might blush and smile shyly, seeing the wink as a sign of interest from an attractive character. This reaction would often be used in romantic - themed fanfics to start a potential relationship between the two characters.
One possible theme could be embarrassment. If it's about an unexpected encounter with a bra, it might lead to an embarrassing situation for him.