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Official English: Cantarella
Uploaded February 21, 2008, with 165,400+ Niconico views and 2,578,000+ Niconico views (revised)
"Cantarella" is an original song by Kurousa-P. It is about an obsessive and intoxicating romance. A man (KAITO) who tries to keep his love for a girl (Miku) hidden and approach the girl in order to make her his. Taking time by the forelock, he gets closer to her, and he finally reveals himself.
KAITO and Miku personify Cesare and Lucrezia Borgia respectively, known for their suggested incestuous relationship and use of poison.
A short remake version of the song was later used as one of KAITO V3's official demos.
This song was later rearranged and uploaded as "Cantarella ~grace edition~". This version was uploaded to YouTube on January 6, 2015. It was made in commemoration of the 5th anniversary of the original song.
Due to the similarities,certain key parts in the story, and other peculiar points of symbolism: this PV is most likely an allusion to the Borgia family legend-the notorious family who was said to have invented the titular poison. KAITO and Miku, play a twisted brother and sister in a story of hidden agendas, bloody murder, romantic passion and forbidden desires; all of which...eventually come to haunt them in the end. However; the end of the PV implies that the tragedy ends on a bittersweet note. 
In this PV KAITO appears to be the Miku's childhood friend and the person who teach her how to play the violin. As his feelings for her grows up, he sets a complex montage to seduce her, appearing as a mysterious masked man that visits each night and in different situations, just for a meeting her.
A relative new Cantarella's PV by Mie. In this PV KAITO and Miku knew since the childhood, having a complex past that binds them. KAITO now as adult is interested in Miku but actually she seems to be engaged or courted by Len. Despite this KAITO sets his plan, getting rid of the suitor and deceiving the girl to make her fall into his trap.
A musical based on this song ran in Shibuya's Space Zero theater in Tokyo from August 3 to August 7, 2011. It was directed by Yukio Ueshima, with Kentarou Kanesaki, Otouto no Ane and Tomohisa Sakou starring in the main roles. The play was also streamed live on Nico Nico Douga.
B's-LOG Comics published a novel called Cantarella -Forbbiden Love is Poison- in September 15, 2011 and a manga adaptation called Cantarella -Immoral Love and End of Poison- in March 1, 2012. The novel was written by Kaoru Ooshi and illustrated by Madarame, the manga was illustrated by Akira Sakamoto.
Ichika made a manga adaptation as well called Cantarella -Poison of Blue- which was published in the digital magazine Nico Nico Age Premium in April 9, 2011 (also available at Yahoo Bookstore). The physical print was released in October 9, 2012.
The first works mentioned are inspired off of the theater play and the later one uses both KAITO and Miku.
Cantarella is a poison or narcotic said to have been used by the Borgia family as their favorite poison in political assassinations. There are many rumors about its nature, preparation and real effects.