Beauty and the Bodyguard - Chapter 5 - Feels Like A Marriage Plan?

As a promise, he went out into the world as an assassin. 3 years later, after becoming the Supreme Assassin, he gave up on everything he had and returned to the ranks of ordinary people. But one day, Kismet, a mystical game created by the National Research and Development had appeared.

Enigmatic hands, spectral frame, and one cunning strike just like the swing of the Grim Reaper. The Supreme Assassin takes on the mystical world, and how his legendary tales sets off, progressively ascending the altar to becoming the best.

- Description from Novelupdates

Leiyin accidentally crossed into the One Piece World, and accidentally obtained the Naruto System.

Madeline Celeste d'Aumont is the first daughter of Duke Aumont and Duchess Louise-Marie d'Aumont who has been engaged to the Second Prince of Pays Des Abrutis since she was a child. The Second Prince decided to marry a woman closer to his own age and broke off the engagement to Madeline who was left with a broken heart and a reputation as being an unwanted woman even before she was an adult. The Duke decided to help his daughter regain her confidence and reputation by sending her to the Academy de L'échec.

Ito Aoi was the son of a Kendo master who ran a dojo in downtown Tokyo. One day while crossing the street to get some Yakitori, Aoi was hit by a delivery truck and woke up as the trash first son of Duke Montfort, Guy de Montfort. Duke Montfort also sent his son to Academy de L'échec only Aoi instantly recognized that he was now living in the world of an Otome Game that his sister loved to play and talk about all the time. Aoi recognizes that Madeline is the villain from the game. Aoi also recalled his sister saying that Madeline was terrible in the game, but is she really?

As childhood sweethearts, they were made to engage under the manipulations of their families. It involved having to to live in the same room and sleep together.

One night, she is accidentally made drunk, and in his delirium, he kisses her only to realize he enjoys it. From time to time, he would sneak a kiss on her, but his “crime” is eventually discovered in an accident. After being accused by her, he says with a smile, “You are my wife, why can’t I kiss you?”

The two people’s daily routine is full of bickering and mutual discrediting of each other.

Although he claims to disparage, he actually cares intensely about her. He feels crazily jealous when she is close to other men, to the point of tying her to his side, so that she becomes exclusively his.

- Description from web