Together Forever's activated ability checks whether the target creature has a counter on it as the ability is activated and as the ability resolves. If the creature loses its counters later in the turn, the delayed triggered ability will still return it to its owner's hand when it dies.

If a creature without counters receives enough -1/-1 counters to reduce its toughness to 0 or less, Together Forever's ability can't be activated before state-based actions put that creature into its owner's graveyard.