Seven is a character from X Finds Out His Value, an old animated video made by Jacknjellify in 2008. They appeared to be X's value. Later on, they appeared in episode 12 of BFB as a recommended character by Cody S, and two shorts.
They also appeared in a short " Thanks for 600,000 subscribers! ", crying because they were forgotten by the channel and want to be on the sub count. In another short, when they were told that they were in the sub count, they kicked into space. Eight considers them as petty, but other than that, very little is known about them.