If you're looking for manga similar to Knife in Her Heart, you might like these titles.
After a successful winter break makeover, Nari is finally ready for her high school debut. But somehow, she ends up friends with the three prettiest girls in school! Follow Nari as she tries to navigate her brand new high school life surrounded by beauties.
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Both webtoon:
Andrew is the only daughter of the famous Grace family. She also has an incurable disease. Due to her family's high status in society, Andrew grew up with no friends save for her maid, Herta, whom she has become intensely infatuated with. Little does she know, her family is responsible for ruining Herta's life. Can Andrew break the hierarchy and be with her? Or will Herta end her suffering first...
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They both have psychological aspects or something like that
One main character has an incurable disease and may die soon, while the other does not.
both have dark pasts.
A girl at my new school asked me out. She's the school's strongest, Yi-Na Kim. Is this an elaborate scheme to punish me? "Can the next challenger wait until tomorrow?" Only the strongest girl can take me! Wait, do I not have a say in this?
"I think about the same things every day. I hope for the same things every day. Perhaps I was born in the wrong dimension; I feel that this is my true world... Maybe the world I was first born in was fake. Because- take a look, this world is beautiful. Only in this world do I feel happy."
The Priyeddie bamboo forest, built to be used anonymously to exchange true feelings. It has been running smoothly as the basis for gossip for three years. But after an MT visit, a sniper fire bombs and a strange rumor began to circulate in the school.
It takes so little to become a target. Hojin only did what he had to do—help out a friend in need. He was trying to save his friend from Jung-yoon, the school’s aggressive bully. Back then, he didn't know that doing the right thing can sometimes lead you to hell. Now the target of vicious bullying, Hojin's desperate to get back to normalcy. He’s had enough. Something inside his soul has broken. He can’t let this carry on. Small ripples of rebellion are forming inside him, and these ripples will soon turn into a tidal wave, a tidal wave that will wipe everything away.
Five beautiful girls are trapped in their school without a clue. "Who kidnapped us...?"