Ulysses Highwater was once a bounty hunter back in the time of the Wild West who's motto was, "We all choose our own hells." Although he was officially a bounty hunter, he wasn't above killing his bounties so long as it meant that he could still collect their bounty, as well as keep their loot. Throughout it all however, Highwater was always searching for a man known as Nathan Drumm, claiming that he couldn't stop until he finally found him. Sadly, Highwater never did find Drumm in life, and so he eventually died and was condemmed to Hell.
Instead of just being another eternally-tormented soul however, Highwater somehow managed to become Hell's bounty hunter and became tasked with dealing with any condemmed souls that sought to upset or escape the torment of Hell. Despite his own damnation, Highwater still held on to his pursuit of Nathan Drumm, certain that he would one day find him in the infinite reaches of Hell.
Ulysses Highwater was presumably and primarily created by Karl Kesel for 2000's Harley Quinn series as a central character for the storyline where Harley went to Hell. Highwater has yet to appear outside of the few issues that he was featured in.
Major Story Arcs
After it became apparent to Etrigan that Harley Quinn was too stubborn to accept her damnation and that her incessant escape attempts would only cause trouble time and again in Hell, he enlisted Highwater to track her down and make sure that she was thwarted in her escape attempts so utterly that she'd have no other option but to finally give up hope. Highwater gladly accepted the task on the small chance that Harley might somehow know where the illusive Nathan Drumm resided.
After several failed attempts at escaping her personal slice of Hell, Harley finally managed to make some real progress, managing to get herself and most of her dead Quinntets so far as crossing the River Styx by hijacking Charon's boat. Unfortunately, Highwater arrived and quickly put an end to their escape by killing them all when he thought they were refusing to tell him the location of Nathan Drumm, even though they in actuality simply had no idea who Drumm was.
However, because Lewis took a bullet for Harley, she managed to live long enough for Highwater to ask her once again if she had any idea where Nathan Drumm resided. Frustrated, Harley started asking just why Highwater had such a vendetta against this Drumm guy, and she actually managed to get her answer in a roundabout way. It turned out that Nathan Drumm was the gay lover of Highwater's son, and Highwater was convinced that Drumm had somehow corrupted his son into becoming gay. Surprised at Highwater's ignorance, Harley firmly stated to Highwater before he shot her that while Hell punished many sins, loving someone was not one of them - meaning that the reason he hadn't found Drumm in Hell was because he just wasn't there.
After being killed by Highwater, Harley was placed into the custody of the demonically-appointed Detective Pettit, who was in charge of her personal slice of Hell. Pettit also attempted to get Harley to forsake her hope, but nothing he tried would work. Eventually Harley presented the desperate Pettit with a deal: find Nathan Drumm for her, and she'd let Pettit beat her.
After getting the info she needed, Harley double-crossed Pettit and attempted to escape once again. This time though, she ran just enough to draw Highwater out again, and when he found her, she asked him if he really wanted to know just where Nathan Drumm was. Harley revealed that Drumm wasn't where Highwater expected him to be, and wherever he was, he was there with Highwater's son. As Harley held out the note with Drumm's location written on it, she asked Highwater whether he loved his son enough to accept him for who he was, and not who he wnated him to be. Unfortunately, as Highwater reached out for the note, the ground split beneath him and he was dragged back into the bowels of Hell, because as Etrigan stated, "Highwater's heart must stay hateful," implying that it was Highwater's hate for Drumm which had made his own personal Hell all these years.
Having beaten Highwater and realizing that Harley would never stop causing trouble in Hell, Etrigan finally cast her out and back to the land of the living.
Highwater was not seen again, presumably having been demoted to just another tormented soul in the bowels of Hell.
Powers and Abilities
In life, Ulysses Highwater was a relentless bounty hunter with considerable tracking skills and marksmanship.
After dying and being condemmed to Hell, Highwater became a demonic shadow of his former self, most likely augmenting him and his abilities. Despite becoming demonically-empowered, Highwater rarely seemed to use anything other than his tracking skills and revolvers. Whether he had any other distinctive abilities was largely unrevealed.