This song has to do with a series of dreams that the main character has pertaining to a sister who does not exist in the real world.
In these dreams, this older, wiser sister teaches the main character not only how to fight and survive, but how it feels to be cared about by someone as well (an experience said character is not too familiar with.) On top of this, since they are both musicians, they often play together while chatting.
The dreams begin to take on a bit of an odd tinge as the story goes on, though. Something seems to be changing, and the truth about them may yet be revealed.

There are a couple leitmotifs from elsewhere in the project buried in here, which relate to the theme.
It's similar in tone to the theme of love -link- , but while that one pertains to all the characters, this one pertains to only these two.

Art by cainethelongshot @ Weasyl