Williams and Finral may be half brothers.

First hint: They both have purple eyes.

Second hint: Both Finrals (real) mother see never seen, nor are Williams parents.

Third hint: Willam's father most likely had a plant base magic and his mother (also Finral's mothers) most likely had spatial magic mixing the two magic attribute's together and you get "Word tree magic"

Forth hint: William stated that his mother died from a cruse which he was then born with on his face. The show makes a joke saying Finral is cursed, but what if it's not a joke, what if Finral was cursed much like William. Only his curse is different then that of William's.