What If the Strongest Final Boss of The Original Story Became The Protagonist's Companion?

The fantasy novel, The Sacred Hero Iris, was adapted into a game. One day, a man was reborn as Euglam, the Empire's strongest third prince and the final boss. To avoid his destined death at the hands of Iris, the enemy kingdom's princess and the game's protagonist, Euglam decided to seek asylum in her kingdom! And finally, he met Iris. But the luxurious kingdom life he expected didn't begin. Instead, he spent his days teaching Iris swordsmanship, acting as her bodyguard, going on dates, and even becoming a father figure to a loli assassin! With his innate Magic Core granting him absolute defense through Magical Barriers, he overwhelms those around him, but…!? "——Well, I'm the strongest, after all." The lovey-dovey story of the final boss who has switched sides and the pure and lovely protagonist (with a violent streak?) begins!!