Born to Don Allen and Meloni Thawne, heir to the legacies of both Flash and Reverse Flash, Bart Allen was raised in the 30th Century in a big VR tank because only a computer could keep up with the speed at which he was aging. His grandmother Iris, wife of Barry Allen, broke him out when he was chronologically two and hit his teens because she was sure Earthgov wasn't interested in curing him, just in studying him. She spirited him away to the twentieth century so his grandfather could help with his speed issues, undershot, and wound up foisting him off on the then-current Flash, Wally West.
As Bart had been fond of saying, the codename he was given – Impulse – wasn't a name, it was a warning. He gained himself a reputation as having a heart of gold but being way more trouble than he was worth to work with. Nevertheless, as the years went on and he was in and out of the Titans and eventually mentored by elder statesman speedster Max Mercury, he grew up while no one was noticing. He was one of the founding members of 'Young Justice', a team of sidekicks and junior heroes, along with Tim Drake, Robin, and Kon-El, Superboy. They were his BFFs!! IT WAS RAD.
Crises continued, as they do, to hammer his reality. Time broke a lot, reality broke a lot, he time travelled, he found out he had a cousin in the Legion of Super-Heroes (XS!) and made friends with some and annoyed the heck out of others, and every time everything blew up he found out more about who he was, who his family was, and what he was capable of. Still, still, no one noticed he was growing up. He was still short, he still had big hair, he still had crazy ADD, and he still pulled pranks and had fun and was never ever grimdarque.
Enter Ra's al Ghul and the Worst Thing Ever. That rat finally got his way, managing to unleash a bioweapon that killed off roughly four fifths of the world population and every single superhero that was on the planet at the time. The only supers who got off the hook were the ones who were time travelling, offplanet, in another dimension, or… whatever, superheroes do a lot of weird things. But the point is it wasn't many. All his friends died.
Even Batman died.
When Bart came back to the present time – because obviously his friends in the Legion and his whole future history were still intact in Hypertime – the bioweapon's remnants had filtered away, cleaned by Mother Earth, just as Ra's had known they would. He's a speedster; it didn't take him long at all to figure out what happened, to figure out who was left. And it didn't take him long to put together, once he put his mind to it, who he needed. More than the Flash, more than Kon or Tim or Max, more than Superman or Zatanna or Wonder Woman– he knew he needed Batman to help him start to put the world back together.
So, having known about Ra's and his habit of resurrection, and its cause, due to both being friends with Robin and lots of crap happening before and being able to remember EVERYTHING, Impulse ran around and around and around the world looking for Ra's or a Lazarus Pit. He found both, and seconds later he was dragging Bruce's corpse in and dumping it into the bubbly goo–
–and Ra's, not at ALL having planned for this, and knowing he was dealing with a speedster, decided to blow it all up.
So *HE* died, Bruce /lived/ because he was crazysauce and rawr and supermighty from being resurrected and not sanity-ridden yet, Bart ran them both out and dropped Bruce in a lake in like Kentucky or something, and while he was waiting for Bruce to simmer down he looked and looked and LOOKED for another Lazarus Pit so he could start bringing other people back to life. He couldn't find any. Like at all.
Basically when he came back to where Batman was heartachily and despondently and grimly and Batmanily pulling himself out of the lake, he was crying and he couldn't stop. But it was just crying, not hysterics. He was able to explain everything. And even if metahumans annoy him, Bruce has, in fact, always had a soft spot for kids who've lost everything, and the explanation of what Bart had done when he came back? It was enough to convince Bruce that the kid was not, in fact, either a complete dip /or/ nearly as unreliable as he frequently portrayed. He'd had an idea, but this was proof. So yeah, he asked Bart if he actually -did- want to work with him to try to fix the world– and of course Bart /did/– and it was Bart who asked if he could keep Tim's boots warm until they could bring him back. If they could. Ever.
It was a WHOLE LOT of 'NO POWERS BART' training. And then fighting weird mutant Mad Max monsters and crime bosses who decided to build Thunderdomes and also Poison Ivy and the Flouronic Man and whatever. They never /did/ find another Lazarus Pit. On the other hand, they were able to establish for realsies that that was in fact bits of Ra's al Ghul all over the place where the pit blew up, so he wouldn't be coming back either, at least.
One seriously bad habit Bart had, and still has, but it was worse at the time, was running through the timestream to just– look. To see his friends again, where they were, in the past. Never to interfere, but always to look. The problem (aside from being morbid and unhealthy) is that when speedsters time travel, the faster they go, the more likely they are to completely accidentally break away from the time stream and into the Speed Force. If there's a crisis going on at the time? *All bets are off*. The last time he tried it? He bounced off the Speed Force, got sidetracked into Skullcrusher Mountain, and wound up hurtling into the City. Right into a lake. Because irony.
