Nemesis

Nemesis is a special agent that can be bought for 50 MM. It can only be placed on towers or bloons. Once placed on a tower, the tower will do 3x more damage to its nemesis (Hypno Monkey/Hypno Bloon). If its placed on a bloon, it will do 3x more damage to its nemesis. It has to be placed 40 times to unlock its pro (2500 MM).

Pro
The pro version lets the tower do 3.5x more damage and also makes the tower take 2x less damage from its nemesis.

Epic
I currently don't know what epic version would do.