Severina is the focus character of the short story Execution, as well as the novel Honorbound, both written by Rachel Harrison.
She was deployed with the 11th Antari Rifles to the Bale stars system to aid the Imperium's war effort in the system. When the regiment was ordered to attack an enemy fortress, the captain of the regiment refused saying it was a suicide mission. He apparently forgot that saying this out loud next to a Commissar is another way of suicide.
Raine *BLAMMED* the captain and then had to calm down the regiment as she had just shot their commanding officer, who was apparently quite popular with the men. She stopped the regimental uprising by stating that the captain had refused the will of the Emperor and that she was a Commissar and shooting people who refused the Emperor's will was her job. She then took command of the regiment and charged them into the fortress, where many of them died but they overcame the defenses and destroyed the fortress.
She believes members of the Regiment still hold a grudge, which considering she shot the commanding officer and then forced the men into a bayonet charge that only a Krieger would willingly commit to, this is probably an accurate assumption.
GeeDubs wants those limited run models to sell, so they made rules for her. The fact that she got rules and an official model when characters like Cain haven't has proven divisive, to say the least.
She's a 36 point Commissar with an AP:-1 Bolt Pistol and a sword. Not a Power Sword, a normal sword with a mere AP:-1. She has the same Aura of Discipline and Summary Execution as normal Commissars, but with one bonus rule: Leading from the Front. If Severina herself is within 1 inch of enemy models, all squads in her Summary Execution range become Fearless instead. This makes her useful in close-range firefights or with melee squads in a support role, but Severina's likely dead if she gets in melee with anyone besides Tau or other Imperial Guard. This is because she doesn't hit hard and she's lacking in defensive gear (there are generic Eldar HQ choices more durable and hard-hitting than her without upgrades) so the list of things that can kill her in melee is very long.