He is portrayed by actor Wil Traval.
Simpson was presumably a good cop in the past. He had enough reason and common sense to stop using Combat Enhancers as it made his rage uncontrollable. However after being "Kilgraved" and attempting to kill Trish Walker, then later to commit suicide he became reckless and obsessed.
Eventually killing Kilgrave became his only goal. He was willing to help Jessica Jones (another victim of the latter) murder the man however he didn't agree with her letting him live as to clear the name of Hope Schlottman and wasn't all that concerned if it may have cost the lives of the few in the process. Despite this Simpson felt undeniable guilt for his forced actions of trying to kill Walker and eventually the two became lovers. Even under the uncontrollable influence of Combat Enhancers, he still gave Trish the chance to walk away as he didn't want to hurt her.
However after a failed attempt on Kilgrave's life which resulted in the death of two of his soldiers and him being severely injured Simpson had no choice but to return to the Combat Enhancers which cured his body but had a traumatic effect on his psyche. On the Enhancers' affect, Simpson was violent, crazed and homicidal. His devotion to murdering Kilgrave went to a degree of almost ruthlessness. Simpson had no concern over breaking the law, murder was no inconvenience to him as he did with Oscar Clemons when he planned to lock Kilgrave away instead of killing him.
Evidently the rage of the Combat Enhancers mixed with his need for revenge against the mind controller was all aimed towards Jessica as Simpson thought that she was too emotionally involved in trying to bring down Kilgrave and acted extremely aggressive toward her, to the point of violently attacking Jones in her apartment.
Simpson takes different colored pills, known as Combat Enhancers, to produce different bodily effects. The primary functions of these pills is to use adrenaline to greatly reduce the physical limiters of the body, allowing hem to exert greater strength, lowered fatigue, lowered reaction to pain and a faster recuperation. These pills include red, for increased adrenaline, white, to keep him balanced, and blue, to bring him down and relax.
- Enhanced Strength: The pills increased his adrenaline, which in turn lowered his physical limiters such as fatigue or pain. This in turn enhanced Simpson's physical strength, as he was able to match Jessica Jones' strength while she was injured. He was strong enough to easily rag doll her and throw her through a wall.
- Enhanced Stamina: Simpson's stamina was increased by the Combat Enhancers; Trish Walker was in disbelief at how quickly Simpson was up and about when anyone else would have been confined to bed rest.
- Regenerative Healing Factor: Due to his increased metabolism & adrenaline, he was able to heal faster than normal. His healing is so advanced that it took him only one night to recover from catastrophic wounds sustained during an explosion.
- Pain Suppression: The pills increased his adrenaline, which in turn lowered his physical limiters such as fatigue or pain. Simpson was able to take massive beatings and injuries without feeling any signs of pain. Jessica Jones pointed out a catastrophic injury on his arm which he did not notice because it did not cause him any pain.
- Expert Marksman: Simpson was able, at a moment's notice, to kill two men who worked for Kozlov with point-blank headshots.
- Expert Combatant: Coupling his paramilitary training with his police skills, Simpson was able to battle the super-strong Jessica Jones (though she had damaged ribs at the time). Simpson won his battle with Trish Walker, who had taken the Combat Enhancers, because of emotional desire, though Walker used her combat training to match him blow for blow.