James Heller was a former US Marine Sergeant and the second Blacklight being. It is not directly known how Heller came to be the way he is today, but one thing is for sure; He has an extreme hatred, and claims that Mercer had killed his wife. What's also certain is that Alex was responsible for giving Heller his powers.
Prior to his infection, Heller was competent and skilled in battle as a result of his military training. He is proficient with the majority of military weapons, ranging from standard machine guns, rapid-fire Gatling guns, and rocket launchers. The abilities granted by his viral infection are divided into 5 categories - Offensive, Defensive, Locomotion, Sensory/Hunting, and Disguise; he has much the of same superhuman and shapeshifting powers as Mercer, as well as many more that he has evolved and/or developed himself.
Heller was first discovered on the Mako colony, in Docking and Observation, where he came upon Adam Jensen and "Knocked him out of his Delusion". He has since joined Marq's group, which has recently reencountered Mercer. After a small struggle between Alex and the group after Llid controlled Mercer briefly, Heller reluctantly allowed Alex back on the team. Since this, he has been considered more powerful than Mercer, since after his brief possession Alex's body rejected Llid's presence when he came to, and began quickly losing Biomass at a fast rate. Alex is now back in a "Reduced state"; Heller still in his highly Mutated, Highly powerful form. It is doubtful that if the two fought in their current states, Mercer would last long.
Heller claims to have once consumed Alex, and "Puked him back out because he didn't tast so good"; however, this might just be something said to signify his Power over Mercer, and it's truth is questionable.
After a while, the Capitol planet was destroyed, and the Third Prototype boarded the Ra Calium. Heller crashed into the Hangar via Space fighter and fought The enemy; To the Death. Even in his Mutated State, Heller was no match for the Third Prototype, and was consumed by him. He didn't even get the chance to see his Daughter alive again, after he saw her at the end of Prototype 2. He was destroyed along with the Third Prototype by Alex Mercer.