Not long ago we had "agelast" = one who never smles. But apparently there is no antonym.
Noverca is Latin for stepmother, and there is an adjective "novercal". But though Latin
"viticus means "stepfather" there is no English word using the root. Seems odd.
Can you think of other such pairs?

It's a great question, wwh, but I just woke up and there's no way my brain's going to turn in that direction so early.

However, somewhere in tsuwm's lexicon he has a word for a phoney, Broadway smile. If he reads here, maybe he'll provide that word. Not exactly constant smiling, but pretty dearned close. x [infinity]

four months later, and still smilin'..

a grinagog is a perpetual grinner.

..and..

an eccedentesiast is a person who fakes a smile, as on television.

Well, slap my face and call me, "Sally"! Thanks, tsuwm!