Credited as Alonzo P. Hawk, he's the main antagonist of the movie. He goes as a banker that initially seeks to demolish the failing Medfield College to put up a new series of apartment complexes, especially after his son Biff is taken off the basketball team for bad grades. When he stumbles upon Brainard's discovery of Flubber after seeing the scientists' flying Model T, he schemes to steal Flubber for monetary gain.
When Brainard tries to take Flubber to government officials, Alonzo manages to discredit him by switching out and stealing the flying Model T. When realizing what he's done, Ned and Betsy go to his house to find the car under the pretense of selling him on the idea of Flubberized shoes. They trick him into trying it out for himself by jumping out the window safely, but he cannot stop bouncing, causing a scene and resulting in the fire department and Medfield's football team working to get him down. He and his men pursue Brainard to his warehouse and engage in a car chase, which results in Hawk's arrest.
In this sequel to The Absent-Minded Professor, Hawk still seeks to tear down the school, especially after Brainard's government deal with Flubber falls through and it remains a government secret. When Brainard's experiments with Flubbergas accidentally break glass all over town, Auld Lang Syne Insurance finds itself in a deep debt from having to pay for repairs. Hawk eventually tracks the incident to Brainard and seeks to seize the Flubbergas as a means of perpetuating a glass installation and insurance scam across the country, though when Brainard denies him, Hawk takes Brainard to court for being responsible for the property damage across town. The case is dismissed when it is discovered that the original Flubbergas testing had the side-effect of functioning as a super-fertilizing "dry rain" that could grow giant vegetables that could more then pay off the damages.
Here, he's also the main antagonist where he goes as a ruthless billionaire who makes a living destroying old buildings (some contains great history) to plan on building skyscrapers and shopping malls. He was going to destroy the Steinmetz Firehouse, home of Mrs. Steinmetz, the aunt of Tennessee Steinmetz, the mechanic of Herbie in The Love Bug, but got foiled by Herbie and many of his friends, including other Volkswagens. In this incarnation, he's renamed Alonzo A. Hawk, probably by mistake.
List of Hawk-inspired characters in other productions
In the 1976 The Shaggy D.A., sequel to 1959The Shaggy Dog, Keenan Wynn portrays another ruthless billionnaire character, named John Slade, which is a lookalike of Hawk in both behavior and look. It remains unknown why Disney didn't simply reuse the name of Alonzo Hawk, as the name is the only difference between Slade and Hawk.
Other Hawk-inspired characters in Disney movies
Wikipedia also lists two other characters who aren't exactly Alonzo Hawk but seem to have been somehow inspired by him; the first being Martin Ridgeway from 1972 Snowball Express, another bad banker played by Keenan Wynn, but less dishonest than Hawk; and the second (and the only animated one) being J. J. Wagstaff from Fluppy Dogs.