Numbuh 621 is a KND member located in Monkey Mountain. You can find him near the entrance to the infected zone. Numbuh 621 has some of Numbuh Two's plans to build a device that could rile up Fuse's Sickle Simians and Scythe Simians. If the invention is a success, these two groups of Fusion monsters will turn against each other instead of attacking heroes. Numbuh 621 is in the Monkey Foothills, and needs someone to help find the materials needed to build this device. His plan includes to mass produce his Loconut which is designed to make Fuse's minions fight each other, but needs your help testing it. His first mission is very unique due to it being the first mission where you have to fight a boss version of Fuse's minions. When the Loconut is broken, Numbuh 621 will send you to Plumber Carol asking her to fix it. She doesn't like the idea and wants to destroy it, but doesn't want to hurt Numbuh 621's feelings.
- Numbuh 621 was originally supposed to be called Numbuh 6021, but the name was programmed wrong.
- His uniform and all other KND operatives uniform is Guide Reward from Ben Tennyson