IF YOU WANT TO READ, REMEMBER THAT THIS WILL BE FULL OF SPOILERS!!
At the begin it was just a strange encounter, but it was very romantic. Takuto had tried to reach the island just swimming, and the final result was that he fainted, even though he reached the shore. Wako and Sugata were occasionally walking through it, and when she saw Takuto, she reached him and made him a mouth-to-mouth resuscitation at once. When he woke up, he finally realized what happened, and the fact that it was Wako the one who kissed him for the first time (mutual thing for her), was not hurting him at all, while she blushed. The only problem, at this point, was that Wako, as South Maiden, was already engaged with Sugata, even if she was still unsure of it. In fact, there were many times in which the girl had more sympathy for Takuto, but she always tried to deny it.
It was Sugata himself the one who found out that both of them where falling in love together, and decided that he would leave the girl to his friend, just if he would win a duel. Takuto's problem was that he couldn't be serious in a fight with a friend, so he ended to lose all the times. Anyway, the "love story" between him and Wako didn't end like this.
In fact, the two of them, became always closer, and they talked a lot about their past and their temporary conditions. Wako started to feel jelous when Takuto was surrounded just by any kind of girl, but even if she wanted to tell him what she felt, she was too shy, and always prefered to be silent... until the day in which she had a second chance and decided to propose herself for the part of the kiss in a piece by Sarina. The kiss was meant to be given to Takuto, and she was serious in this; but she was not the only one: the boy, in fact, invited a very close friend of him, and decided to introduce Wako as his girlfriend, or at least... this was the thing that she always said, but it was never specified! XD The scene of the kiss, in my opinion, was really beautiful, but it lacked of emphasis... ç_ç
When Wako celebrated her birthday, the idea to cook for her was Takuto's, and, above all, his one was the only gift she would receive, if he won't suggest something that Sugata could give her as a gift too. In the last part of the final battle, Takuto will finally free Wako from her seal and save her and the Earth.
That's it, in brief... I really didn't like how their development was treated at the end of the series... :/ I hope in a special episode, where it will be explained what the characters made after that Takuto and Sugata were still floating over the Earth.