Thirty-Six is a Numberblock made of 36 blocks. They were mentioned on We're Going on a Square Hunt as 6x6, but officially appeared later in The Big One as a compound Numberblock. This continues until in More To Explore where their figured-out form appeared as a drawing.
Compound form: Thirty-Six is a Thirty and a Six combined.
Figured-out form: It is unknown if it's official, well in Six's drawing, they have a pink step-shaped mask because they are the 8th step squad. They also have square eyes because 36 is 6x6.