Evil Series- The Full Story (from sqeaky93)

Rin's Daughter of Evil:
Rin is the ruler of the Yellow Kingdom and falls in love with Kaito, Prince of the Blue Kingdom, but he falls in love with Miku, a girl from the neighboring Green Kingdom, she is not a princess. At the same time, Rin's twin brother and servant, Len falls in love with Miku at first sight. Rin find out that the one she loves, Kaito, and her brother, Len, are in love with Miku, who is a mere village girl. Rin gets jealous and orders Len to kill Miku and destroy the Green Kingdom. Len being the obedient servant and loving brother he is follows her orders and kills Miku. Kaito later finds out and sets after Rin for revenge. He goes to Meiko, who is a village woman in the Yellow Kingdom, who wants to rebel against Rin for killing her lover and for the state of the kingdom. A large army of citizens set after Rin, but Len decides to help her escape by switching places with her because they are twins. Len is captured in Rin's place and put to death by the guillotine. Rin, disguised as Len goes and sees her brother's execution and realized the error of her ways.

Miku's Daughter of Green:
Miku is a village girl in the Green Kingdom. The King has planned to have a war with the Yellow Kingdom but finds that he needs a trigger. He finds out that the reining princess of the Yellow Kingdom, Rin, is in love with the prince of the Blue Kingdom, Kaito. Here is where Miku enters, the King used her as bait to lure Kaito to fall in love with her, but at the same time Len falls in love with her. This causes Rin to get jealous and order Miku to be killed, which she is. You can probably guess what happens from there.
As for whether or not Miku loved Kaito or Len is up to the listener. I believe that Miku loved Len because in none of the songs is it said that Miku fell in love with Kaito. According to Miku's song, she fell in love with Len and was only baiting Kaito because she was ordered to by the King. When Len goes to kill her, she stabs herself and thanks him. Although she wishes that Len would have killed her instead of killing herself.

Meiko's Daughter of Vengance:
Meiko is a village girl of the Yellow Kingdom. The Kingdom has been reduced into a wasteland because Rin takes their fortune for her own luxuries. Meiko's lover goes to Rin to plead for her mercy, but instead Rin has him killed by the guillotine. After that, Meiko has sworn revenge over Rin. Later on, after news of Miku's death and the destruction of the Green Kingdom spreads, Kaito goes to Meiko for revenge over Miku's death. They form an alliance and go to capture Rin. When Meiko, Kaito and their army make it to the thrown room Meiko notices that Len has switched places with Rin. She informs Kaito and Len meets the guillotine in Rin's place. After that, Meiko feels guilty and confused for what she did, but admits it was for the sake of her and Kaito's revenge.

Kaito's Prince of Blue:
Kaito is a prince of the Blue Kingdom who fell in love with a village girl from the Green Kingdom called Miku. Their love is forbidden because of their class and he sneaks out at night to be with her. Later he sees that the Green Kingdom has been destroyed and Miku has been killed by the servant(Len) of the princess from the Yellow Kingdom(Rin). Kaito wants revenge and goes to Meiko who also wants revenge for her murdered lover. When going to capture Rin, instead they learned that she switched places with her servant Len and has escaped. Len is killed, but Kaito is still unsatisfied and kills himself to be with Miku in the afterlife.