If an opponent would put one or more counters on a permanent or player, they put half that many [...] rounded down.
Does the "*/*" of my creature behave then as a "+X/+X", which would be halved by my opponent's spell, or is "*/*" my base power/toughness? It seems to me the "*" is a value defining the base power/toughness rather than counters added to a base of "0/0".
But my interpretation is obviously biased by my desire to win. I expected to find answers to this in Rules 208 or 613, but did not.