Dorsal Dragons are so named because they have large fins along their backs that can be raised and lowered at will. They use these fins to regulate body temperature; they raise them to cool off when they become overheated. The bright colors on the fins double as a means of attracting mates.

Dragons are highly-intelligent reptilian creatures that—from a human perspective, at least—appear to live forever. Many different varieties of dragon exist, each with their own unique qualities, habitats, and behavior. Adolescence in dragons is usually marked by the growth of a hatchling’s wings, although not all breeds of dragons grow wings and some breeds have other traits that indicate the beginning of maturation. In Galsreim, dragons and humans coexist peacefully.

Len Kagamine is probably the most devoted brother in history. His younger sister Rin was chosen to be the ruler of her Cave; not wishing to be apart from her, he took the job of her servant. She was a cruel queen, and jealousy drove her to order the death of an innocent dragoness. Len had fallen in love with that dragoness at first sight, but his desire to protect and obey his sister led him to do as she had asked. The other dragons of the Cave were not so complacent, however, and they captured Rin's castle with the intention of having her executed for what she had done. Desperate to save her despite all her evildoing, Len took advantage of their identical appearances and switched places with Rin so that she could escape unharmed. He himself was later released when his captors realised that they had the wrong dragon.

Now the two siblings live in a Cave far away from the one that Rin used to rule. They are members of the Vocaloid clan, and are never happier than when singing together.