All y’all westoids reeing, crying, and screaming but this is a classic trope in Japanese literature that’s existed since before most of your countries even took shape.
It started from Murasaki Shikibu, a lady-in-waiting of Empress Shoushi of the Heian era. She wrote one of the first novels in the world, during the Heian period at the start of the 11th century, The Tale of Genji, which featured the titular main character, Prince Hikaru Genji taking care of a young girl and teaching her until she became an adult, at which point they wed.
Now, distasteful as this may seem to all of you whining troglodytes, there is a massive cultural difference here, and this isn’t seen nearly as negatively in Japanese literature or media as is seen by westoids. This is also mostly exclusive to literature or media; Japanese people are not grooming young girls into becoming idealized brides any more than westerners are.
So stop being bitches about ‘grooming this’ and ‘grooming that.’ In these stories, the girls have their own agency, and choose to fall in love of their own will. The Tale of Genji itself was written by a woman, and the novel was very popular amongst courtiers of the Heian regardless of sex, the men and the women. You’re all dismissing that for some women, being raised by a benevolent father figure and falling in love with said father figure is a fantasy of theirs, as well.