The "Big Cube" (also known as "Friend" in the game files and its avatar) is a supporting character in Just Shapes & Beats' Story Mode. It is first encountered during the Paradise chapter and, as its name implies, it is a close friend of the Player Character. It is encountered throughout the game with the exception of the Tutorial world but plays an important role in only Paradise and Industry.

More facts here: https://justshapesandbeats.fandom.com/wiki/The_Big_Cube