Hari (FL) threatened the senior with the CCTV recording + his messages. Hari gave Yeonju (the senior's girlfriend) the CCTV recording. They broke up. Later, Hari, Yeonju, Yuha and Hajin (Hari's brother) went to a trip.
That day was Hari's birthday. Yuha gave her a pair of earrings. Later at dawn, Hari found out that Yeonju left the room. Yeonju was going to commit suicide. She jump off from into the ocean and Hari tried to save her but the wave was too big and she drown with Yeonju. Before she lost consciousness, she saw Yuha was saving Yeonju. Of course she's heartbroken.
When she woke up, she decided to cut off ties with Yuha and Yeonju. That time Yuha was so frustrated. His actions towards Yeonju also changed. He said "were you always this kind of person?". He didn't answered Yeonju's calls and also changed his house passcode.
Yuha thought Hari changed because she found her first love. He was jealous when he saw them together.