I think I got the background story of Elvin down now.
He was a stray kitten that was taken in by Scat Cat (making him his adopted son) and was raised around the alley cat band. As he grew, so did his love of jazz and he occasionally still plays drums with the boys while having his own adventures in Paris and abroad. He feels a particular close bond with O'Malley and calls him his "Uncle Tommy." Growing up around real jazz cats, it's easy to see that the alley cats are super proud of their prodigy.

Drawn by  thelupinprincess