Seven is a recurring recommended character, number, and a recurring character in the subscriber milestone videos.
Seven, as their name suggests, is a number seven. They were pitch black in "X Finds Out His Value", but they were changed to a lighter black in subsequent appearances.
Seven only cares about being in the spotlight, according to Eight. Seven is a whiny person, due to them crying over not being in the sub count (and therefore not in the spotlight) in "Thanks for 600,000 subscribers!". As well as being selfish, stubborn and uncaring to an extent, shown by them being unwilling to help the other numbers catch Six and to leave the sub count so Eight can take their place in "Thanks for 800,000 subscribers!".
They first appear sitting on a bench in "X Finds Out His Value". They later appear dancing at the end of the video after Four and X find out the latter's value is the same as Seven's.
They reappear once again in "The Four is Lava" as a recommended character near the end of the episode where Clock seems to be talking to himself. This is their fourth appearance in a row.
Seven's biggest roles have been in the subscriber milestone videos. They appear in "Thanks for 600,000 subscribers!", where they are crying because they want to be on the sub count, but 6 is instead.
In "Thanks for 800,000 subscribers!", Seven was seen not wanting to leave the sub count. They were later kicked off by Six, akin to what he did in the previous milestone video. Eight soon takes his spot afterwards.