Kodama of the East Tree

The chaining is what turns green's usual "deep" ceiling on free permanents into something wide instead. Most permanents-to-battlefield effects fire once: a single trigger, a single free spell. This one refires on every subsequent enter, so as long as your natural plays keep landing permanents (a fetchland cracking, a token-maker spitting out bodies, another creature's ETB dropping a second thing into play), each of those becomes a fresh trigger, and each trigger drops another permanent of equal or lesser mana value out of your hand. The exclusion clause (permanents put onto the battlefield with this ability don't themselves retrigger it) is what keeps the thing from looping on its own output: the free permanents are payoffs, not fuel, so the width of the chain is capped by how many enters your other cards generate rather than by what this card puts down. That is why it behaves less like a value creature and more like a ramp payoff that pays you in board state instead of mana, converting any burst of cheap permanents into a simultaneous tumble of everything smaller in your hand. The Partner tag frames it as a builder's tool rather than a solo threat, an open invitation to pair it with a second commander whose enters density feeds the trigger. The 6/6 with reach is almost incidental, a serviceable body doing its real work once the enters start stacking.