Celestial Unicorn

The trigger fires on the event, not the amount: gaining ten life at once and gaining one life ten times over are worth very different numbers of counters, and the card cares only about how often, never how much. That single word ("Whenever," not "for each") is what sorts the decks that want this from the decks that don't. A stream of small, repeatable gains (lifelink pings, per-turn drains, a Soul Warden trickle) grows the body counter by counter, while a single fat lifegain refund does almost nothing for it. This is the exact template Ajani's Pridemate established: frequency of gains over their size, the shape the most durable lifegain payoffs have always taken. The upside is that each counter is permanent and free: the body climbs without demanding more mana or a card in hand, so an established life-matters engine turns this into a finisher that snowballs on autopilot. The cost is that it does nothing the turn it lands. On an empty board it is a plain 3/2 whose ability sits idle until the lifegain is already flowing, so its threat level is borrowed entirely from the rest of the deck rather than generated on its own. Built as a common-rarity payoff for an incidental-lifegain shell, it wants to arrive after the small triggers have set the table, not before.