Jack Krauser is a relatively minor character from the Resident Evil video game series. First appearing in Resident Evil 4, little was revealed about his past except that he had once worked alongside Leon Kennedy as a member of the U.S. Secret Service. His history would be explored in The Darkside Chronicles during Operation: Javier.
Krauser was first introduced to the world of biological warfare while working as a US SOCOM agent alongside Leon S. Kennedy during a mission in South America. Although he was a seasoned soldier and bore the scars to prove it, he knew nothing about the horrors of Bio-Organic Weapons or weaponized viruses that could turn human beings into monsters. He would soon learn the terrible truth when he and Leon discovered that their target, a drug lord known as Javier Hidalgo, had acquired numerous B.O.W.'s and samples of the T-virus on the black market and was using them to spread fear and chaos throughout the region.
As the mission progressed, Jack learned about Leon's previous experiences with bio-weapons during the Raccoon City Destruction Incident and how he was recruited by SOCOM to begin with. While initially taken aback by the sight of zombies and irregular mutant creatures, he soon learned to adapt to the situation and he and Leon succeeded in their mission to stop Javier's operation. However, during the incident, Krauser became enamoured by the possibility that such man-made viruses could be beneficial. He had suffered severe injury to his arm during the mission and it would not recover on its own, which resulted in him being dismissed from field duty. Krauser, determined to regain his purpose in life, dug up as much information as he could about B.O.W.'s and the viruses used to create them from SOCOM's files. During this search, he came across the name "Albert Wesker", a dangerous individual known to have worked in bio-weapons development as a researcher for Umbrella. Determined to seek out Wesker, Krauser made arrangements to disappear from SOCOM; he faked his death, making it appear that he had died in a helicopter crash, and then went in search of Wesker without his ex-superiors being any the wiser.
In late 2004, Krauser resurfaced in Europe as an apparent member of the Los Illuminados, a group of religious extremists which had access to a deadly weaponized organism known as Las Plagas. The cult intended to use the mind-controlling parasites as part of a plan to take over the entire world, a plan that Krauser agreed to play a part in. He kidnapped Ashley Graham, daughter of the then-President of the United States, offering her to the Illuminados as a Plaga host that the cult could use in order to infiltrate the highest level of the American government. In exchange for this service, the cult's leader Osmund Saddler offered Krauser newfound power in the form of a dominant-strain Plaga. The mutant parasite regenerated Krauser's injured arm and made him stronger and faster than ever before.
However, Krauser's service to Saddler was a farce; his only real intention for following Saddler's orders was to gain a Plaga sample that he could deliver to Wesker, who had instructed both him and Ada Wong to infiltrate the Illuminados and acquire a dominant Plaga to aid Wesker in his ambition to restore Umbrella under his leadership. Since Leon Kennedy had been sent to rescue Ashley, Krauser took this as fate and confronted his old comrade. The two fought inside a set of ruins on Saddler's island stronghold, with Krauser using his knowledge of the terrain to his advantage, attacking Leon from carefully selected vantage points and even ambushing the agent with a series of robotic drones.
In order to progress beyond the ruins, Leon needed three key stones, two of which were hidden around the maze while Krauser held on to the last one. The time came when Leon fought Krauser face-to-face and Krauser revealed just how powerful - and inhuman - he had become. Krauser's left arm transformed into a horrific organic blade which allowed him to shield himself from Leon's shots and attack with superhuman precision and speed. Even with his body enhanced by the Plaga, Krauser was unable to kill Leon and found himself defeated.
Though beaten, Krauser actually survived his fight with Leon. He then decided to go after Ada Wong, who had betrayed Wesker by helping Leon to survive the dangers of the island. Though severely injured, Krauser was still a dangerous foe and attacked Ada atop one of the base's towers. He failed to defeat Ada as well, and she managed to kill him for good.
Krauser often came across as cold and aloof; he was an experienced soldier and had seen much hardship in his time on the battlefield which left him grizzled and bitter towards the world. Believing that he couldn't fit in to everyday society, he engrossed himself in his work, caring about nothing but the next mission. After the injury to his arm, Krauser felt like he had lost his purpose in life, but would find new purpose in aiding Wesker's plans to bring about a new order to the world.
As an ordinary human, Krauser was in peak physical condition and highly experienced in armed and unarmed combat. His favourite weapon was a combat knife and he was skilled with both wielding it in melee combat and throwing. He was also skilled with handguns, submachine guns, explosives and archery.
After being injected with a dominant-strain Plaga, Krauser gained enhanced strength, agility, reflexes and stamina. Most notably, he could transform his left arm into a twisted organic blade that could be used both for offence and defence. The bone structure of his mutant arm was so hard that it could deflect bullets and could shield his whole torso and head. The blade could easily cut through flesh and bone and Krauser attempted to impale Leon with this grotesque limb when the two fought each other. During the fight, however, Krauser could only shield his upper body with his arm, leaving his legs vulnerable to attack. Leon was able to defeat Krauser by shooting his legs and then striking the body while he was stunned.