Snide is a weasel and former member of the Kremling Krew before he was kicked out prior to the events of Donkey Kong 64.
Snide was once King K. Rool's chief technician, responsible for the creation of the Blast-o-Matic and presumably other machines the Kremlings have used over the years. After Snide completed construction of the Blast-o-Matic, K. Rool began to become increasingly convinced that Snide was going to betray him. At the height of this paranoia, K. Rool confiscated Snide's work and kicked him out of the Kremling Krew.
After this expulsion, Snide moved to Donkey Kong Island where he set up a headquarters called Snide's H.Q..
Snide aids Donkey Kong and his Kong pals in their adventure to defeat K. Rool by collecting the Blueprints they can obtain from the soldier Kremlings called Kasplats. For every Blueprint the Kongs give Snide, he will reward them with a Golden Banana (in an increasingly bizarre way); also, for each Blueprint given, the Kongs gain one more minute to shut down the Blast-o-Matic in Hideout Helm - giving Snide all forty Blueprints would give the Kongs fifty minutes (as opposed to the 10 default minutes) to shut down the weapon. Also, if the Kongs give Snide all forty Blueprints, they will be able to play Donkey Kong 64's various minigames at his headquarters.
After the Hideout Helm stage, Snide isn't seen again.