Kid Quantum/Jazmin Cullen
Jazmin Cullen was born with the same minor time-manipulation abilities as her brother, James. After he was killed on the Legion's first mission from his over-reliance on his power-augmenting stasis belt, she resented both him for bringing shame to their family and the Legion for getting him killed.
As a result, soon after his death, she underwent a dangerous operation to increase her natural power to a level where she wouldn't require outside aids to be useful. She then joined Xanthu's home-grown super-team, the Amazers, with a chip on her shoulder.
As a result of this, when Star Boy was drafted to join the Legion, she arranged for a mechanic friend to lock his entertainment system to play Star Boy's least favourite type of music - Durlan space-shanties - for the whole trip. At full volume. On infinite loop. This would lead to him eventually crashing into a mining colony and nearly being killed. Whether or not she ever found this out is unclear.
She first encountered the Legion directly when Star Boy was summoned home to rejoin the team, and brought several other Legionnaires with him in an attempt to convince Xanthu's government to let him remain with the Legion. Ordered to fix a Legion-Amazer fight for the right for Star Boy to stay with the Legion, she complied until Lamar, the diplomat who arranged the fix, made an offhand comment that the Amazers beating the Legion would repair the damage done by her brother's death. Disgusted by his lack of appreciation for James' sacrifice, she walked off and the Legionnaires won in short order.
She was later a part of the Task Force who engaged Mordru. Saddened over the loss of Atom'X, she played a critical part, slowing Mordru for much of the battle until he finally noticed her, when only Invisible Kid's intervention saved her. Exhausted, she played no further part in the fight and was hospitalised for a short time afterward. She then returned to Xanthu, mentioning that she wanted to help plan Atom'X's memorial.
The next time she encountered a Legionnaire, however, was when Star Boy and Monstress, fearing the worst with communications with Xanthu lost, returned home to find the planet being invaded. As the Khunds arrived to kill the fleeing invaders, Star Boy off-handedly offered her a place in the Legion, and she accepted to show up the "bunch of pampered show-offs". She was officially voted in soon after, and ended up spilling her life story to Karate Kid, saying how she was finally starting to grieve for her brother after all that had happened.
When the Time Trapper transformed most of the team into replicas of the Silver Age team soon after, she was one of the exceptions, the time link of her power having "impressed" the Trapper.
She served with the Legion for some time until the Blight arrived, gradually accepting the team more and growing close to Element Lad and Umbra, until, after the Blight attack, she was one of the Legionnaires lost in the Rift, getting caught in it even as she and Element Lad managed to seal it.
Over the course of their time stranded in the Second Galaxy, Kid Quantum emerged as one of the team's most reliable members. She helped rally the team during their initial escape from the Progeny, calming Chameleon and Umbra down and befriending Shikari. Fiercely dedicated to the idea that Element Lad had survived, she was the one who realised that the Kwai's most recent "feral star" was actually Wildfire's disembodied energy. Brainiac 5 would also explain to her how her powers worked.
However, her close friendship with Element Lad would, like Monstress, eventually blind her to his insanity after he was revealed as the Progenitor, and it was only after Monstress' death that she was able to see what he had become.
After their return to UP space, Kid Quantum was on the Legion team, under Cosmic Boy's leadership, that delayed Ra's al Ghul long enough for Legion World to be brought in to counterbalance the crashing Moon. Ra's complimented her during that battle by saying that she was the only Legionnaire in that group who had any credible super-powers.
She also cut off most of her hair.
After the Ra's al Ghul conflict, Cosmic Boy quietly recommended Kid Quantum as Legion leader, causing her to win - much to her surprise.
She was quickly thrown into the deep end when Robotica, under Computo's leadership, took over earth, having already devastated Jazmin's homeworld, Xanthu. She performed adequately as leader, although it was Kinetix and Brainiac 5 who eventually defeated Computo, while M'Onel suggested Warworld as the Roboticans' new home in the aftermath. However, her contribution cannot be overlooked, as it was she who rallied the Legionnaires and gathered UP and AP forces to help contain the Robotican forces.
Thereafter, she supervised Timber Wolf's first day of membership, and entered a relationship with Cos. She also kept Cub in a quantum bubble to retard his accelerated aging. She, along with Violet, Superboy and a returned Live Wire, were instrumental in defeating Darkseid/Post-Zero Hour when the villain attempted to erase time.
Powers and abilities
Jazmin's powers expanded hugely over her time with the Legion as she came to better understand her capabilites. Initially, she was able only to slow time around a person or object, but could later also speed up time, fire energy bolts, fly without the use of a flight ring and - under special circumstances involving temporal fluxes - time-travel under her own power, taking others with her in the process. Her powers could also shield herself and others from harm.
Some practical applications of her powers include accelerating decay to destroy or disintegrate objects, and hold others in stasis. However, she mentioned that she was unable to completely stop time, only slow it down.