"Kimi ni Sasagu Fantasia" was first uploaded on May 31st 2010; a VOCAROCK song, with a fantasy-like themed. It was PolyphonicBranch's first work using Kagamine Len, and is among some of his notable work. The song is relatively popular, and has reached Nico Hall of fame, with currently over 200,000 views to date.
According to PolyphonicBranch's blog on the song, in between the climax of the song, she added Rin as a part of the chorus (also at the part where an illustration of Len and Rin together in the video). The reason to that, was to symbolizes Rin's "prayer" for Len's journey in the song's "plot".
During the Kagamine Appends' release, Kimi ni Sasagu Fantasia became one of the demos for Len's Append voicebanks "Cold" and "Serious". It was also labeled by Karen-T, making it purchasable in Listen Japan, iTunes store, TSUTAYA RECORDS, and more.
The song tells a story of Len, in his adventure / journey, bearing unbearable pain, though he chose to move forward despite all that. Len wanted to find Rin's smile and happiness, thus the title being "fantasia dedicated to you".