Kid Quantum/James Cullen/Reboot
As a child, James Cullen was hugely competitive, often fighting with his sister Jazmin over trivial things. When his weak power over time emerged, he obtained a "stasis belt" which could amplify it to useful levels, and joined the Amazers. Arrogant to a fault, on the day of the annual Planetary Championship, he was pulled out against his will the day before the final (which he was due to contest with Atmos and Star Boy) to be Xanthu's representative in the Legion of Super-Heroes.
Legion and Death
He proceeded to annoy most of his teammates with his ego, and strained against Leviathan's autocratic rule. Ignoring his direction to wear his stasis belt outside his transuit, he stormed on ahead in the Legion's first fight, against the insectoid Tangleweb. With his belt power running low and the transuit interfering with its operation, his overconfidence would prove fatal, as the belt failed at a critical moment, and Tangleweb snapped his neck and threw him away.
His death caused Leviathan to stand down as Legion leader in favour of Cosmic Boy, who had to talk him out of quitting completely.
His corpse would subsequently be destroyed by a bomb placed in his coffin at his funeral, which included something which activated the stasis belt to trap the Legionnaires in place for the explosion, and only XS' speed saved her teammates.
His death would cause both his sister and the Xanthuian government deep embarrassment. His sister would undergo dangerous surgery of her own design to enhance her powers to levels which did not require external enhancement and joined the Amazers in her own right, also as Kid Quantum. The Xanthuian government would attempt to force her to fix a match between the Amazers and several Legionnaires over who could keep Star Boy as a member, which she stopped doing when the prefect present made an insensitive remark over his death. She would later join the Legion, and rise to become leader.
It is also likely that his death caused Section 3.1.(i) of the Legion Constitution to be written, requiring that a member must "have at least one genuine super-power not activated, assisted, or powered by extrinsic means".
Powers and abilities
With his stasis belt, James could create a stasis field of limited size. Without, his power was negligible.