in actually it all started when Kimito was in his early teen years he was in rebellious stage and he ran away from home when he ran into another kid that look like a lot like him, the boy name was Kazuto Kujou the real older brother of Miyuki Kujou. Kazuto came up the idea to switch identity to their families’ think it was it would be a fun prank, and thus Kazuto started to live in Kimito house while Kimito did the same thing in Kazuto mansion. while was living in the Kujou mansion, he learned several things from the "upper class" style (dance, self defense, etc ...) and meet many people of that lifestyle (here it is when he meets Reiko Mother which he became a candidate to marry Reiko) course that Kujou family learned that the child that their having is not Kazuto but they continued with the "act", and of course that's where also met Miyuki, Miyuki was a very sharp but shy little girl she was not very close to Kazuto reason being that he was very cold to her, so when Kimito saved her from being bullied at her school she started to his Kimito as her new “big brother”. After a few months Kimito was living with Kujou families and his relationship with Miyuki became closer than ever but when Kimito was about to leave the mansion Miyuki started to cry and wish that Kimito won’t leave her but Kimito told her that he always be her big brother, after a Christmas party when Kimito met Aika he got a fever which made him lose his memory about the Kujou family and his relationship with Miyuki. However Miyuki still remain her love to Kimito and she wanted to be close with him even if he doesn't remember the time they spent together, when Kimito joked about that he like maids she decided to became his maid and when the school needed a "Shomin sample" she insisted that they should pick Kimito despite being related to one of the most powerful and rich families.
As for Eri, she was a gloomy and lonely kid and was just moved in to Kimito neighborhood when Kimito asked her to play with her with their DS, they started to enjoy themselves and when they started to watch anime she started to dub a voice from a character from an anime their watching Kimito told her that he liked her voice acting, and thus she started to develop a crush with Kimito and also abandoning her old persona, after some time she made some friends and some bully stopped bullying her. during high school she became a voice actrees and during valentine she wanted to confess her feelings to Kimito however she heard a conversation between Kimito and some guys he hang out he was being asked that if he was going out with her, he said that he was never going go out with her and he received chocolate from some girls and thus she became aggressive towards Kimito because of that.