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Izuku is a princess and Katsuki is a prince. They both belong to different kingdoms and since they were born her father and his mother agreed to an arranged marriage between the two. After the two of them turned seven their parents would bring them anytime they needed to see each other to allow the children to get to know each other. They would sometimes get along but most of the time they would bicker and argue. As they got older the arguing turned into teasing and turning more playful interactions, they would spar during sword fighting lessons, share interests for their future and go on 'adventures' together (just roaming around the nearby forests).
One day Katuski is invited to The Midoriya Kingdom by Izuku's father to discuss the agreement. When he learns the truth he struggles to see her in the same way. He suspected that these feelings were building but he didn't truly believe them, he just wanted his friend. But he didn't think that Izuku would be coming to stay a week late, the reason is that her kingdom is at threat of The Todoroki Kingdom and if anything was to happen to the king his daughter needs to be safe. The next few months Izuku is kept safe within The Bakugou Kingdom and she and Katsuki grow a lot closer. One day in the evening after dinner the two of them were sparring when the castle was attacked by people looking for Izuku, the two of them are able to defend themselves until their leader makes himself known as Dabi and they begin to struggle. During their fight he lets their intentions slip about "taking Izuku to take the kingdom much easier", this shocks Katsuki enough for Dabi to leave a gash in his arm. This distracts Izuku enough for someone to send a sleeping dart to her neck causing her to pass out. Katsuki tries to keep fighting but it’s struggling and Dadi is close to killing him but then an archer appears, killing Dabi and the others trying to take Izuku. The archer relieves himself as Shoto Todoroki, a son of King Todoroki but he has left his family after he learned his father's plan to either force Izuku into a marriage with one of his sons or to use her against her father to hand over his kingdom. Shoto works with Katuski and Izuku to stop this fight between kingdoms. A couple of weeks later Shoto discusses that Katsuki is planning to marry Izuku, Shoto agrees that it would be best to protect Izuku.