DNAngel

Daisuke Niwa is a 14 year-old-boy who has a special DNA due to his genes. Whenever he sees or thinks of his crush, Risa Harada, he turns into Dark. Dark was created by "The Black Wings," a very large piece of artwork, along with Krad. Krad is the complete opposite of Dark, and is from the Hikari family like Dark is to the Niwa family.
Later on, after being rejected by Risa, Daisuke starts to notice Riku more often then later on he falls in love with her. Riku first thought Daisuke was a idiot but then she realises that she likes Daisuke. Riku had a crush on Daisuke near the beginning, way before Daisuke liked her. But until Daisuke is able to tell Riku about his condition and she can love both of him and Dark, he'll keep transforming. Unfortunately Riku, who had her first kiss stolen by Dark, now thinks Dark as a "pervert" and hates him for kissing her. She loves Daisuke and cares about him a lot, but on the other hand she doesn't know Daisuke's other half is Dark and she hates Dark for kissing her.
Satoshi Hiwatari is a student at the same school that the Harada twins and Daisuke. He goes to keep an eye on Daisuke because he is the Commander of the police force and heads the investigation on Dark. Satoshi has Krad in him just like how Daisuke has Dark in him.
Interestingly enough, Risa falls in love with Dark, and proceeds to do all sort of absurd things to get him to go out with her, which he eventually does. She has also kissed him on more than one occasion, and it is hinted that Dark loves Risa; but Dark claims that he does not love her, and that (according to the anime) Risa's grandmother, Rika, is the only woman he has ever loved. In the manga, Dark's reasons for rejecting Risa seem to be so that she is not hurt by the fact that he is immortal and not really human, and therefore cannot participate in a functional relationship. As expected, Risa doesn't really listen to him and continues to try to get him to go out with her.