Storm Monkey is an unlockable playable character in Bloons Super Monkey 2 Mobile. He's unlocked after you clear all the stages in Frozen Fields.
Storm Monkey used to be a normal Super Monkey, but that changed when he was passing through Helium Heights one day and was ambushed by a gang of ZOMGs (although Super Monkey says he heard it was Red Bloons). Storm Monkey tried to lose them by flying into a storm cloud, but it turned out the bloons were herding him into a trap; The Golem was lying in wait inside the cloud and captured Storm Monkey when he arrived. Storm Monkey was then placed in an electric cage, where he stayed for so long that by the time Super Monkey destroyed the Golem and freed Storm Monkey, the electrostatic energy from the cage's bars had infused him with the power to control the weather (specifically, lightning and wind). Dr. Monkey gave Storm Monkey some weapons to help him control his new powers. After this, Storm Monkey decided to team up with Super Monkey and Ice Monkey to fight the bloons in Helium Heights.
Storm Monkey's weaponry is nearly identical to that of Super Monkey's; the only difference is that Storm Monkey uses Storm Weapons in place of the Magic Weapons. His left and right arm weapons are lightning attacks that can't pop Glass Bloons, and his core weapons are wind attacks that don't affect Lead Bloons.