Ben transfroms into Brainstorm in Pier Pressure when he had to fight a Galavanic Mechomorph double named Ship. Ben first selected to transform into Jetray but it switched to Brainstrorm after the pier shook, which makes Brainstrorm the first accidental transformation.
Brainstorm's powers involve superior intelligence comparable to that of Grey Matter. Unlike Grey Matter, Brainstorm has offensive capability from his brain in the form of electricity, forcefield projection/control, and mind bursts. Unfortunately, doing so requires Ben to expose his brain that is usually covered with his resiliant shell, which is vulnerable to attack.
He takes heavy damage if someone directly hits his brain. If water hits him, he would get shocked by his own electricity.