What if the strongest final boss in the original becomes a companion of the main character?

Synopsis

The original fantasy novel that was adapted into a game, "The Holy Hero Iris." One day, a man found himself reincarnated as the story's strongest final boss, the Empire's Third Prince, Ugram. To avoid his destiny of being killed by the protagonist, the enemy nation's Princess Iris, Ugram decides to seek asylum in her kingdom! And finally, he manages to meet Iris. However, the luxurious kingdom life☆ he was expecting does not begin... Between acting as Iris's swordsmanship instructor and bodyguard, going on dates, and becoming a father figure to a loli assassin, his everyday life is chaotic! He overwhelms his surroundings with his absolute defense, "Magic Barrier," which is powered by his innate "Magic Core," but...?! "Well, it is because I am the strongest." The lovey-dovey, cheat-filled life of a final boss who defected to an enemy nation and a pure, lovely protagonist (violent type?) is now beginning!