Both are set in the '90s before moving to present day at the end and have a nostalgic feel and a school setting. Both have sweet and somewhat poignant romances in which the leads somehow miss being together (permanently in 20CG, temporarily in ARRTI). One has a sad ending, the other happy.

Both are Republican Era mini dramas where the FL is out for revenge for her family and is initially distrusting of but involved with the ML as part of her scheme. She later finds out the ML and she knew each other as children and he recognized her first.

Both are historical mini dramas that are about a strong FL ending up in a marriage with a sick, delicate ML who has a hidden identity and agenda.

Both FLs time travel into the past to save the ML who has died in the present. It's then revealed that the ML was secretly in love with her a long time before she believes they first met.

Both dramas are about one of the leads suddenly dying and the other attempting through time travel to save their life.

Both FLs time travel to their school days in an effort to save someone.
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Both dramas are about one of the leads suddenly dying and the other time traveling back to the past in an effort to save their life.

Both dramas are about one of the leads suddenly dying and the other time traveling back to their school days in an effort to save their life.

Both FLs time travel back to their school days.
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Both FLs travel into the past to save the life of the ML.
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Both FLs time travel into the past and their school days in order to save the ML who appears to have committed suicide.

Both are Republican Era mini dramas in which the FL is a self aware character in a video game who accidentally gets transferred to another game and must set things back on track.