I Was a Sword When I Reincarnated (WN) - Chapter 870

When The Cute Kunoichis Spied on Their Teacher's Secret is a light novel written by Kazuma Kamachi and illustrated by Ryuushi Tajima.

The story takes place in a world where Japan never ended their isolationist phase, and agents from foreign powers are always threatening to steal the country's advanced technology. In order to combat this threat there has been a resurgence of ninjas that use Strategic Martial Arts, techniques that combine cutting-edge modern technology with a traditional foundation of martial arts and ninjutsu. The story follows Souha Sugiyado, a 17-year old ninja instructor and former ninja prodigy who was forced to retire from active duty by the wounds he suffered in his last mission. Souha's four students are all kunoichi who have all gained the rank of Elite Ninja, the highest official rank available to ninja. However, one day two Shogunate ninja approach him with the news that his four students have gone rogue and have stolen a portable modular nuclear reactor, and thus they ask for his help in bringing his students to justice.

Shi Qing Xia is practically an angel.

Kind, sweet-hearted, and good at her studies, she was admitted to the top university in Taipei.

However, the living expenses for the metropolitan city were far greater than she had ever imagined, so she started to work as a private tutor for wealthy families to make ends meet.

On her first day of work, she met Zhao Yi Bo and Zhao Yi Xuan, a pair of neglected 16-year-old genius twins whose prosperous and wealthy parents heartlessly abandoned in Taiwan. Shi Qing Xia found them to be gentle, compassionate, friendly, clever, mysterious, pitiful and attractive.

But appearances can be very deceiving…

Because behind those sweet smiles…

Deeply twisted and cruel demons lurked, waiting to rip apart the wings of this angel, condemning her to an eternity of conflicted bittersweet suffering — a rollercoaster ride of a heart-wrenching, twisted, and tragic romance more extreme than what someone so innocent could possibly imagine.

- Description from Novelupdates

Dying at a young age after a disappointing life. Mat thought he hit the jackpot when he got reincarnated on a tropical island as a baby. Here, exploring mysterious lands and learning magic are a reality. The world of Elydes is beautiful and magical, wild and savage. While the Guide is impartial to all, people are not. Mat has the lowest starting point possible, and those like him rarely go far. To not let his humble birth define his life, he'll need to find the determination to grow stronger and delve into the mysteries of Elydes.

He, without doubt hates her.
What did she do to deserve his hate?
The person who she treated well like a brother did not even bother to notice it...

Years passed…

He needs to get married.
He needed a wife.
So she offered herself.
It was a marriage that will benefit both parties he said.