Numbuh 206 was featured in Operation: F.U.G.I.T.I.V.E..
He was a Kids Next Door operative who was scheduled to be decommissioned after he turned 13, causing it to fall to Earth and strand the people aboard on a beach near the , which he then headed towards, apparently working for its residents dandelion while she, Numbuh 3 and Numbuh 5 stormed the mansion where it is revealed hat "he" is actually Numbuh 362, who had been spying on the Delightful Children when she was mistakenly captured by the decommissioning squad.
Thus, Numbuh 206 (presumably) was never decommissioned, was never mentioned since this incident.
An operative wearing 206's outfit was seen running past Numbuhs 362, 86, 2, 3, and 4 in the Kids Next Door Moonbase in Operation: Z.E.R.O. after two thirds of the Sectors on Earth were Senior Citi-zombiefied.