Alex Mercer is the playable character of video game, Prototype.
As a human
Once a human, Alex works for a company known as Gentek. He has no empathy, as evidenced when he is questioned of how does he feel of creating a deadly virus to be 10x deadlier, he replies that he "wasn't paid to feel".
One day, he is pursued by Blackwatch, a military unit and took a vial of the virus as "insurance". Alex is cornered by the team in Penn Station. As a result, he drops the vial to the ground, hoping to release the virus and put Blackwatch and Gentek in a untenable situation. He is shot to death afterwards.
As a virus
His body is shown being brought to Gentek for dissection. the virus, having merged with his body, became an independent being based on Alex, codenamed Zeus. Zeus woke up, having no memory of how did he end up there, and tries to escape. Just then, Blackwatch corners him and shot him, but he survives.
Alex gets into contact with his younger sister, Dana Mercer, into helping him discover the truth. She even directed him to Karen Parker, his ex-girlfriend, to create a cure for the virus. However, the latter is possibly forced to work for Blackwatch as the genetic material he got for her is used to make a weapon against him. This is removed from him with help from Dr. Bradley Ragland.
Just then, Dana gets kidnapped by a Leader Hunter under orders of Elizabeth Greene. Alex has managed to locate her, and gets into a fight, but ended up creating the Supreme Hunter when he injected her with the same weapon. He saves Dana, nevertheless.
Next, Alex gets into contact with a mysterious informer, who knows everything about him. He reveals that he is the virus, and that Alex is just his role.
The contact lead him into a confrontation with Director Raymond McMullen, his ex-boss at Gentek, to get information. The latter reveals of how Alex Mercer is responsible for the plague by unleashing the virus in New York before having died at Penn Station.
Alex finally meets the contact, Captain Robert Cross, the specialist he fought early on. He reveals information on getting Colonel Taggert as a ticket to get into the Reagan to stop General Peter Randall from destroying Manhattan with a nuclear missile. When they got there, Alex absorbs Randall, and gets attacked by Cross, who is revealed to be the Supreme Hunter in disguise.
Alex defeats the monster, and puts the missile into the river. He fails to escape as the explosion takes him, too. However, his remains absorbed a crow, regenerating himself.
Prototype 2Alex has witnessed a soldier named James Heller taking down the infected. What he didn't know is that he does that only to die fighting them.
When Heller becomes wounded, Alex infected him with the Blacklight Virus, giving him the powers he would have. This leads him to have a vendetta for Alex as he knows he is responsible for the death of his wife and daughter.
The original Alex was a cold monster with no empathy who ultimately unleashed the virus out of pure spite. His actions and demeanor is likely due to his troubled past, which made him cold individual, both paranoid and sociopathic. On the top of that, he seemingly doesn't care about his younger sister, Dana, and is seemingly only using her as a pawn in his plans as well.
But in truth however, Alex is not completely heartless as he seemed; actually, he deeply trusts and cares for his younger sister Dana, having been the only real paternal figure for her, and had what can be assumed to be a loving relationship with Karen Parker.
The virus creation that merged with him, Zeus, was ultimately a better creature, displaying acts of genuine heroism and was horrified and disgusted with the original's actions. Even he is more caring to Dana after infected by Blacklight virus, as she is the only one who can help him to uncover his past. However, his change is likely because his fractured humanity, as his mind was taken over by the virus itself since infection, that explain his amnesia upon his resurrection as virus monster. In addition, he rarely takes human cost into account, being able to murder scores of humans without remorse or hesitation despite his heroism.
Unfortunately, after repeated betrayals and mishaps during his time away from NYZ after the initial outbreak, Alex decided that stopping the outbreak was a mistake and started a new one himself to usher in a new race; he claimed he was making a better world by doing so. In his new role as the leader of the Infected, Alex pursued his goal of spreading the virus in a far more active and aggressive manner than Elizabeth Greene ever did, even inserting spies within both Blackwatch and Gentek and using their resources and scientists to do so.