The question should rather be: "What could it have been?" - It never made it into an existing machine. According to some Internet lore, Kelly would have been the A3000+'s/AA3000's RAMDAC (digital video memory to analog RGB converter chip) that Commodore was initially planning to develop themselves.
According to the same lore, during the development of Kelly, Commodore encountered a jungle of already existing patented technology that they would have had to navigate that it was decided to use an already existing 3PP (Analog Devices ADV7120KP30) off-the-shelf circuit instead.
Here's a reference: http://obligement.free.fr/articles/chipsetamiga.php#kelly