Ug was an alien bounty hunter from outer space with shapeshifting abilities, allowing him to take on the form of presumably any being he desired. He was shown to be capable of transforming into humans and Krites, although after the first movie he kept his human image until his eventual death.
Years later, at some point after Critters 2, Ug became an Executive Officer of the Enforcement that used to employ him, Lee, and Charlie McFadden as bounty hunters. He employed Storm Trooper-like henchmen during this time.
In the first two Critters movies, Ug was generally portrayed as a good being. He was tough and very dedicated to his job, but by the end of the first movie, he began to show compassion towards the Browns (mainly Brad) and even offered a communication device in case he was ever needed again.
In Critters 2, he displayed a more lively personality, smiling and speaking a lot more than his first appearance.
He only appeared in a flashback sequence and a small cameo at the end of Critters 3, which didn't allow him to show much of his personality at the time. He still called Charlie his friend, but he was much more to-the-point and business-oriented than prior.
Critters 4 saw him return in person with a completely different personality than he had ever shown, being driven purely by his selfish desire and greed. He had no regard for human life, openly killing Albert with no remorse, shouting angry demands, and even willing to kill his former friend Charlie in order to get what he wanted. When asked why he was so different by Charlie, he simply said, "things change, Charlie...things change." Many fans have theorized that this drastic personality change came about due to the loss of Lee, but so far there has been no official explanation from anyone involved in the franchise.