Windy was a smart student but she was so shy that it was so hard for her to talk with people without feeling awkward or embarrassed. She had crush on kris but never confessed because of her shyness and afraid of getting rejected.

Windy friend sana knew about it and wanted to help her friend out so she locked windy in library with Kris when the school was going to close so nobody can distrub them.
Because they were stuck together for a long time, they started talking with each other so both won't feel bored.

Kris enthusiastic and straightforward nature let windy speak her heart out and she no longer feel shy around him because he made her feel comfortable.

She musterd her courage to propose Kris taking the opportunity. Though Kris was stunned, he accepted her proposal because he also liked her.
Windy was no longer a shy and timid person after meeting with joyful Kris.