Karios, Master Spellfuser

Whenever you cast an instant, you may pay {u}{u}. If you do, exile it as it resolves.
Whenever you cast an instant, you may pay {r}{r}. If you do, you may cast an instant card from your hand without paying its mana cost and return it to your hand as it resolves. Until it resolves, instants you cast from exile return to exile as they resolve.
Whenever you cast an instant, you may choose any number of cards exiled by Karios and cast them without paying their mana cost.