The truth about the secret between Na In Woo and Park Min Young was shown in the most recent episode.
Yoo Ji Hyuk (played by Na In Woo) told Kang Ji Won (played by Park Min Young) about the blue heart in the TVN drama "Marry My Husband."
A surprising change of events happened at the end of episode 6 when Yoo Ji Hyuk recognized BTS songs like "Dynamite" and "Spring Day," which were shown in the future.
Then, in the most recent show, show 7, it came out that both Kang Ji Won and Yoo Ji Hyuk had died and gone back to the past.
At the beginning of the show, Kang Ji Won talked about her strange plans for her husband Park Min Hwan and her best friend Jung Soo Min to get married. Later in the show, Yoo Ji Hyuk talked about a sad event that happened to him in the future: he had been to Kang Ji Won's funeral. He told the story, "It was sad." "A lot of people had regrets," he said, talking about the sad feelings he had been going through at the time.
People found out that Yoo Ji Hyuk had helped Kang Ji Won as soon as he got back to the past to make sure she could start over.
Yoo Ji Hyuk felt bad that he didn't take the chance when it came up, so he chose to stay with Kang Ji Won and give his life to protect her.
Yoo Ji Hyuk shows Kang Ji Won the blue heart mark on his neck at the end of episode 7. He then asks Ji Won, "Didn't you say your dad drew blue hearts on your allowance?" and Ji Won replies, "Your dad's allowance."
He keeps making it clear that he wants to help Kang Ji Won get payback by saying, "So you can use me all you want." It's all right.
"I don't have to spend it, right?" Kang Ji Won asks. I only want to talk to it. I also want my income to be pleased. What would you like to do in your second life?" "I want to be happy," she tells him. I want to be content by myself. with my power. I'm going to be glad.
Source(1)