I remember Curley Greenleaf the half elf druid out of the first couple of Gord the rogue books. One of the other supporting characters was Chert the barbarian.
Gord is the main character. Curley makes a couple of extended "cameo" appearances (i.e. he's present for a few chapters). As far as I know, he has not appeared in any books other than the Gord saga.
Curley is the NPC party member to Gord's PC status, so-to-speak, although Curley was actually played by Gary as a PC. The full roster of Gord books is:
Saga of Old City 1984
Artifact of Evil 1986
Night Arrant 1987
Sea of Death 1987
City of Hawks 1987
Come Endless Darkness 1988
Dance of Demons 1988
with a smattering of short stories appearing in Dragon 100 and a couple of other collections.
Curley Greenleaf--who according to Gary's notes was actually a character played by Gary in his campaign--appeared in the Greyhawk Adventures books "Saga of Old City" and "Artifact of Evil", then later in the Gord the Rogue Novel "Come Endless Darkness".
He's a supporting character, since the books center around Gord. He appears only in a few chapters of the first novel, but plays a major role in the other two novels.