In Zero's debut episode, he makes things disappear. Six returns everything to normal and brings Zero home.
Zero and One together can make ten.
He acts like a baby because hes one of the youngest numberjacks along with 1 and 2.
Despite being a baby, he is shown to be quite smart for his age, as shown in Zero the Hero when he was able to use the Brain Gain Machine. He can also say his own name, but doesn't talk properly yet due to his age.