David, Perceptive Apprentice

When David enters the battlefield, choose
an opponent to become your mentor.
Whenever your mentor casts a non-creature spell,
draw a card.
Whenever you cast a spell, your mentor creates a treasure token.
{b}{b}{u}{u}, {t}: Exile a card from your mentor's graveyard. You may cast it and spend mana as
though it were mana of any color to cast it for as
long as it remains exiled.