One (1) is a purple Numberjack with Blue-green eyes. In The Trouble With Nothing, she was referred to as a female.
She is best friends with Zero and they have even caused trouble together in Tens Moments.
When the Numbertaker was about to suck up Six in One Won, Numberjacks Three, Four and Five prepared an emergency launch to save him and used Brain Gain to make her scarily big as suggested by Agent 94.
In One More Time, she made everyone and everything that was going under bridges one more.
At the beginning of 1, 2, 3, Go!, she falls asleep nonstop during Three's new game, in which the Numberjacks say their number in order while their Buddy Blocks jump.
Although she is very young she has immense intelligence for her age, as demonstrated by her ability to use the Brain Gain machine and the fact that she knows the concept of "one more".
However, she is still very young, so she is not able to use much logic or speak in full sentences, she is described as being "still in her own world".