Leviathan

Gim Allon, a resident of Mars, graduated from the science Police Academy. During the high speed pursuit of a suspect, and accident involving a radioactive meteorite gave him the ability to become giant at will. The government of Mars named him Martian representative to the Legion of Super Heroes, where he took the code name Leviathan.

The United Planets appointed Allon as a team leader, but after a disastrous mission that resulted in the death of Kid Quantum, he relinquished control to Cosmic Boy and became deputy leader. Later, after his team mate Shrinking violet became possessed by the omnipotent Emerald Eye, Leviathan sacrificed his life to defeat the evil Doctor Regulus. He received a hero's funeral on Shangalla.

In an alternate Legion timeline, Gim Allon is a member of a race of Antarctic giants who has the ability to shrink to normal human size. He goes by the name of Colossal Boy, despite his protests that "Micro Lad" is more accurate.