Numbuh 621 is a KND member located on Monkey Mountain. He can be found near the entrance to the infected zone. Numbuh 621 has some of Numbuh Two's plans for 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 needs someone to help find the materials needed to build this device. His plan is to mass produce his "Loconut," which is designed to make Fuse's minions fight each other, but needs the hero's help testing it. His first mission is special, being the first mission in which the hero has to fight a boss version of Fuse's minions. When the Loconut is broken, Numbuh 621 will send the hero to Plumber Carol to ask 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 incorrectly.
- His uniform (identical to all KND operatives's uniforms) is the Guide Reward from Ben Tennyson.