"hexproof + unblockable" is a good combination for "invisible", but a really annoying one to play against. So here we have unblockable + partial hexproof, which is hopefully still well worth having, but not as overpowering.

And fits the cloak quite well -- it's harder to aim a spell at someone in a cloak, not impossible.

I would reword as "Whenever enchanted creature becomes the target of a spell or ability, counter that spell or ability unless its controller pays " to avoid confusion about abilities that don't have costs like most triggered abilities.