Regina Kasteen is the commanding officer of the Valhallan 597th Ice Warriors, a renowned regiment of the Astra Militarum. Her rise to command was forged in tragedy; originally a Captain in the all-female Valhallan 296th, she was thrust into leadership when a brutal Tyranid invasion on the world of Corania wiped out all of her superior officers. Showcasing remarkable tactical prowess and survival instincts, she successfully pulled her surviving troopers out of the meat grinder.

Following the devastating campaign, the understrength 296th was forcibly merged with the equally battered, all-male Valhallan 301st. The merger was a recipe for mutiny, but Kasteen secured supreme command of the newly christened 597th over her counterpart, Major Ruput Broklaw, strictly on a bureaucratic technicality—she had held her Captain's rank for three days longer than him.

Despite her initial self-doubt regarding her youth and experience, Kasteen quickly proved to be an exceptional officer. Alongside Major Broklaw (who became her fiercely loyal executive officer) and the famous Commissar Ciaphas Cain, she molded the fractured, co-ed unit into one of the most cohesive, lethal, and tightly-knit regiments in the Imperium. Pragmatic, blunt, and fiercely protective of her "family" of troopers, Kasteen has led the 597th through countless grueling campaigns across the galaxy, earning the absolute respect of her soldiers and the highest praise from the Hero of the Imperium himself.