【Kagamine Len】Love Dark Tower [English subs] 

Translation by Terracannon876‎
I tried making a decent PV with the concept art and the art from the rain dream tower PV.
edit: I just realized how bad the text looks
Summary of the whole story from the wiki:
Rin's View:
A song about two courtesans, a girl that lived as an orphan, Rin, and the daughter of a former samurai, Miku, in pre-modern Japan. As children, they promised to be together forever, however they are now working in brothels and eventually become rivals. A wealthy man named Len comes to the brothels and enters relationships with various courtesans. Rin notices the man visiting the brothels frequently and falls in love with him but is heartbroken when she discovers her friend, Miku, in his arms one day.
In her heartbreak, Rin realizes that she and Miku have truly become rivals in love and that their dream to walk on together someday may just be a fantasy. Len leaves for another country and promises to Miku as well as others, that he will return. However, Miku dies from an illness that has frequently plagued her. As Miku dies, she asks for Rin to convey her feelings to Len as she believed his promise to return. Rin waits for the man to return but understands that she is ultimately waiting for no one and wishes everything had been a dream.
While some parts of the song lyrics alone can be vague, the official book by Team OS, Music Reference Volume 1(ひとしずく×やま△楽曲設定資料集vol.1), goes into further details about the relationships between Rin, Miku, and Len. It also further explains details such that Miku had been ill for the majority of her time in the brothel, the reason why Len ultimately leaves is for work in another country for his family's business, and that Len had made this promise to various other courtesans.
Len's View:
Len is born the third son of a wealthy family. The first son is the kind eldest brother named Gackpo and the second son is a carefree painter named Kaito. Len has been in love with his eldest brother's fiancee, Luka, since his childhood. Luka and Gakupo have been arranged to be married since their childhood. Unable to have her, Len seeks out relationships with various courtesans.
While in his affair with Miku, Len begins to see an older woman named Meiko. However, Meiko does not take Len seriously knowing he sees various courtesans despite him telling her to consider a relationship. In the end, Len ends up leaving for a different country due to his family's work.
Music Reference Volume 1(ひとしずく×やま△楽曲設定資料集vol.1) by Team OS and illustrated by Suzunosuke has further details of Koi Yami Rou and confirms that the woman in the main illustration as well as concept for a PV illustrations is indeed Meiko. It further explains Len's hidden love for Luka and how he began relationships with various courtesans including but not limited to MEIKO and Miku.
Something Important: Len never met Rin