Albert Wesker is the primary antagonist of the Resident Evil series. He has made plot appearances in Resident Evil, Resident Evil Zero, Resident Evil: The Umbrella Chronicles, Resident Evil: Code Veronica X, Resident Evil 4 and Resident Evil 5.
 Plot Contributions
 Resident Evil
Wesker was sent with the rest of the STARS members to investigate a mansion in the Arklay Mountains in Resident Evil. Wesker makes occasional appearances throughout the game to move the story along. It is eventually revealed that the Umbrella Corporation is behind the T-Virus, which broke loose in the mansion and turned the inhabitants into zombies. It is also revealed that Wesker was really working for Umbrella and planned to betray them as well, before he releases the Tyrant, and is supposedly killed.
 Resident Evil Code: Veronica
It is revealed in Code: Veronica, that Wesker is still alive. He injected himself with a new virus he obtained from William Birkin before being killed by the Tyrant. The virus also gave Wesker super-human powers. Throughout Code: Veronica, Wesker attempts to steal the T-Veronica Virus from Alexia Ashford. He fails to get the virus from her, but is able to retrieve some from the body of Steve Burnside.
 Resident Evil 4
In Resident Evil 4, Wesker appears in Separate Ways, which was only available on the PS2 version. Wesker uses Ada Wong to retrieve the Las Plagas sample from Osmund Sadler. Ada manages to retrieve the sample at the end of Resident Evil 4, and Wesker seems assured that the sample will aid the revival of Umbrella.
He's also playable in the mini-game called The Mercenaries in Resident Evil 4
 Resident Evil 5
Wesker returned in Resident Evil 5 as the primary antagonist. Years before the game begins, working with Excella Gione and Tricell, Wesker reopened the previously-abandoned African Umbrella research facility and began work on creating a virus more powerful than anything Umbrella had ever made. Thus, the Uroboros virus was born. Planning on saturating the planet with his new virus, Wesker hoped to create a "superior race" of evolved humans and become their god.
However there was one flaw in the Uroboros virus. It was extremely poisonous and killed all of the human test subjects that were infected with it. Luckily for Wesker, a solution would be found in none other than Jill Valentine. After being kidnapped by Tricell, Wesker originally intended on using her as a doomed Uroboros test subject in order to get revenge against Chris Redfield. But inside Jill's body were powerful T-Virus antibodies that had lay dormant inside her since the Raccoon City incident. Using these antibodies, Wesker was able to make the Uroboros virus less deadly.
Wesker then loaded the virus into missiles and was ready to spread it across the world. But Chris Redfield and Sheva Alomar soon showed up to foil his plans.
 Personal History
Wesker was once friends with William Birkin, and both of them worked with Umbrella. They both aided in Dr Marcus's assasination.