From Halopedia, the Halo wiki
John Butkus was born in 1976 in Toronto, Ontario, Canada. He was hired by Bungie in 2003 and has worked there as an animator ever since. Not much is (definitively) known about him, though his Bungie.net profile is extensive.
He was credited as an animator for Halo 2, and still worked at Bungie to work on Halo 3.
Butkus left Bungie in April 2008, to work at Blizzard Entertainment.
Private Butkis from Another Day at the Beach was most likely named after him.