Can Fury of the Lion target a knelt character?

Absolutely. It's always possible (unless a card says otherwise) to choose to kneel a knelt character or to choose to stand a standing character. The only time you can't is when doing so is required to pay a cost.

Keep in mind that if an effect is along the lines of "Choose and kneel a character. Then, do nasty things to that character." then choosing a knelt character will cause the "then" part to not activate.

All of that is tangential to your question, as FotL doesn't have either of those constructions. The answer to your question is yes.