Alex Mercer

Alex Mercer was a biological engineer who was hired by Gentek, a biochemical company, because of his skill in genetics. He became the chief of the Blacklight project. The project's goal was to engineer weaponized viruses to help the government. Through the group's work, they were able to create a Blacklight virus that has the capability to replicate the living cells of another organism. Blackwatch, a viral-containment special-forces group, tried to muzzle Gentek employees and launched a purge upon scientists working for the biochemical company. Mercer was one of those scientists. He tried to flee but was caught by Blackwatch. Out of anger, he smashed a vial of Blacklight virus onto the ground, starting a biological outbreak that spread throughout Manhattan. Blackwatch operatives shot Alex to death and left his body. The virus enveloped Mercer's body reviving him and granting him supernatural powers. Alex is now stricken with amnesia and tries to find out what happened to him and solve the Gentek conspiracy.

Battle
See Omnicube1's blog: http://deadliestfiction.wikia.com/wiki/User_blog:Omnicube1/Alex_Mercer_%28PROTOTYPE%29_vs._Cole_MacGrath_%28InFAMOUS_game_series%29

Thanks to prototype.wikia.com for the facts and pictures

Battle vs Achilles (by Leolab)
Achilles somehow winds up in modern-day New York, face-to-face with Alex Mercer. The infected human quickly slams the ground with his hammerfists, and Achilles dodges the blow. Recognizing the threat, he pulls out his sword and knocks Mercer to the ground with his shield.

Mercer’s arms transform into claws, and he slashes at Achilles, most of his blows being halted by the warrior’s shield. Achilles tries to get blows of his own in, but his sword is deflected by Mercer’s claws. His sword eventually gets caught in his opponent’s claws, and Mercer twists his arm away, disarming Achilles. Mercer transforms his arm into a blade, and starts trying to slice through Achilles’ shield. The shield withstands the onslaught, and Mercer pulls back, making his arm into a whipfist. Achilles decides not to close the distance, and throws his javelin as Mercer launches the fist. The javelin shatters on impact, and the whipfist continues, but its speed is reduced enough that it glances off of Achilles’ shield. The Argive warrior then takes out his spear. Mercer’s arm morphs into his blade again, and he charges Achilles. Achilles stabs at Mercer, his larger reach keeping his opponent at bay. He eventually scores a hit, the spear driving into Mercer’s normal arm. Clutching his injured limb, Mercer decides on a new strategy to maximize his agility advantage. He runs up the building, out of Achilles’ sight. The warrior, startled, looks around for his quarry and spots him as he leaps into the air. He brings his blade down in a karate chop, missing Achilles but slicing his spear in two. Mercer then disguises himself and disappears into the crowd. Achilles moves through the frightened people, and tracks Alex by his behavior. Alex jumps over a building and into an alley, ignored by the watching Blackwatch Soldiers and Marines. Achilles, dumbfounded by the soldiers incompetence, walks into the alleyway.

He turns the corner, putting his shield up just as Mercer launches is whipfist. In his weakened state, the fist bounces off of Achilles’ shield. The warrior closes in, and repeatedly bashes Mercer with his shield. Mercer is reduced to a bloody pulp, staining the shield and walls. Achilles walks back to where the fight started, picks up his spare javelin and spear and walks away, confusedly hearing the soldiers from before talking about Zeus being down.