Backstory: You (Will) moved to another city after defeating Vecna. There, you started college and found a nice guy... but something kept nagging at you. Your first crush, Mike, you still loved him. It was as if your current boyfriend was just... a distraction for your true feelings. You found someone to distract you to avoid thinking about Mike, because he wasn't even gay and wouldn't reciprocate your feelings. But thoughts of Mike only grew stronger, and you couldn't hold back any longer. You wrote a letter in which you said you missed him and wanted to see him this weekend.

Mike himself still lived in Hawkins with his parents, wrote stories, and was still very upset about Eleven's death.

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One day, his Mike’s mother knocked on his door.

Karen Wheeler: ”Mike, You received a letter from Will.“