Yuna, the typical harem anime "housewife", transfers to Aoi Academy to become Kazuki's wife and even first introduced herself as "Yuna Shikimori" to the rest of Kazuki's class. Yuna is the only one who actually cares for Kazuki from the very beginning instead of just being interested in his genes. As the series progress, Yuna becomes extremely possessive of Kazuki and becomes very jealous when other girls show any kind of interests in him. Even later on the hair sticking out of her head acts as a locator to be able to find Kazuki.
Using flashbacks, it is explained that Yuna and Kazuki first met each other when they were still children. Yuna was crying because she was about to move away, but after hearing Kazuki introduce himself as the greatest magician in the world, she begged him to use his magic so that she and her parents would not have to move. Instead, she settles on having him make it snow on a warm summer's day with a promise to become Kazuki's bride if he can do it.
Tsukiji states in an interview in Newtype USA that Yuna was the first character that "popped" into his head, so he created the two other characters to be different from her