Stun counters replace untapping for any reason, including players untapping tapped permanents during their untap steps.

If a permanent has more than one stun counter on it, only one will be removed each time it would become untapped.

Abilities that trigger when a permanent "becomes untapped" won't trigger if a stun counter is removed instead.

If a tapped permanent with a stun counter on it would become untapped, a stun counter will be removed from it instead. This is a replacement effect.

If untapping a permanent is part of a cost (such as that of Halo Fountain's first ability), you may pay that cost by "untapping" a tapped permanent with a stun counter on it. The stun counter will be removed and the creature will remain tapped. However, the cost will still be paid.

If a spell's kicker cost was paid, the spell is "kicked."

If you put a permanent with a kicker ability onto the battlefield without casting it, you can't kick it.

The kicker ability doesn't let you pay a kicker cost more than once.

To determine a spell's total cost, start with the mana cost (or an alternative cost if another card's effect allows you to pay one instead), add any cost increases (such as kicker), then apply any cost reductions. The spell's mana value remains unchanged, no matter what the total cost to cast it was.

If a card or token enters as a copy of a permanent, the new permanent isn't kicked, even if the original was.

If you copy a kicked spell on the stack, the copy is also kicked. If the copied spell is a permanent spell, the token the copy of that spell becomes when it enters is also kicked.