The persist creature does not have a -1/-1 counter on it.
Steps
Activate Ashnod's Altar by sacrificing the persist creature, adding ({C} magic symbol) ({C} magic symbol) .
When the creature dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
Activate Hexavus' second ability by paying ({1} magic symbol) and removing the -1/-1 counter from the persist creature, putting a +1/+1 counter on Hexavus.