Willy Deever is a minor character in Batman: The Enemy Within. A regular patron at the Stacked Deck Bar, he was an associate of Frank Dumfree and "John Doe". When the latter became the Joker, Willy would help him either bring the Agency to "justice" or in his revenge schemes against Bruce Wayne.
A craftsman by trade, Willy Deever held several jobs within Gotham City, including the Ace Chemicals. However, he was continually fired for drinking on the job. Willy was also a regular patron at the Stacked Deck in the Otisburg district. At some point, he became acquainted with "John Doe" and began attending his group therapy sessions. It's possible he know Frank Dumfree and Selina Kyle prior meeting John.
The day after the Pact's attacks on Gotham, Willy attended the group therapy at the Stacked Deck with John. During the session, Willy made a drunken rant about Batman and Bruce Wayne, not knowing the latter was standing behind him. Upon realizing he was, Willy stopped his tirade. John then left him to talk with Bruce, leaving Willy a bottle of "medicine" underneath his chair.
After finishing his "medicine", Willy reproached the two, threatening them with a knife. Thinking that Bruce was bothering John, he threatened the billionaire. After he either threatened him further or turned onto, Bruce dealt with Willy, either violently or by bribing him. Regardless, Bruce and John left him at the bar.
When John Doe became the Joker, Willy was one of the first he recruited to join his operation. It's possible Willy was the one who gave him access to the now abandoned Ace Chemicals factory. Depending on whether Joker becomes a vigilante or criminal, the following possibilities will occur to Willy.
If John becomes the vigilante Joker, Willy helps develop a number of his equipment, including his Jokerangs. Several weeks after Joker's attack on the Gotham Bridge, Willy joins Frank Dumfree and Lauren in setting up a confrontation with the Agency's operatives. Though interrogated, they all refuse to give up Joker. After an unknown amount of time, Special Agent Roger Harrison decides to execute them, beginning with Willy.
Saved by Batman, Willy watched as the vigilante confronts the group and the subsequent appearance of Joker. He, like Frank and Lauren, are not involved in the subsequent fight between the Agency and the vigilantes. After the fight, Willy celebrates with Joker and the gang, though flees when Bane is deployed to fight him. When Harrison loses control of Bane, Willy attacks him, demanding that he calls of the criminal. Depending on whether Batman chooses to save him or Harrison, Willy can die at Bane's hands. If Batman saved Willy, it is currently unknown whether he is captured by the Agency or GCPD, or escapes to return to Joker. Regardless of his survival, Batman would later use Willy's employment records to track Joker to Ace Chemicals.
If John becomes the criminal Joker, Willy becomes one of his henchmen. It is unknown whether he helped create any of his and Harley Quinn's equipment. During their attack on Wayne Enterprises, Willy accompanies them and personally announces Joker's arrival. Before the criminal releases Project LOTUS, he helps place a gasmask over Bruce's head. He escapes with Joker, Quinn and the gang, possibly releasing more of the virus throughout the building. It is very likely Willy was arrested along with the rest of the gang.
Willy seemed to loyal to people he was friends with, refusing to inform on them. He also was willing to give his life to stop criminals. However, when drunk, he acted aggressively and was easy to aggravate. He was also easily manipulated, particularly by the Joker.