First of all, he can't be the target of spells and abilities your opponents control, meaning he's pretty tough for your opponent to deal with
This sets off two kinds of triggers: Rite of Passage rushes to the aid of the stricken Spirit, putting a lot more than 3 counters on it in return
Ajani, Inspiring Leader Apostle of Invasion Aragorn, Hornburg Hero Arcade Cabinet Ascendant Acolyte Assault on Osgiliath Big Mother Mouser Biogenic Upgrade Bright-Palm, Soul Awakener Bristly Bill, Spine Sower Byrke, Long Ear of the Law Captain Marvel, Apex Avenger Chocobo Knights Court of Garenbrig Death Kiss Dragonsguard Elite Drizzt Do'Urden Elvish Vatkeeper Evolution Vat Fangs of Kalonia Ferrafor, Young Yew Fractal Harness Gavel of the Righteous Gilder Bairn Grand Master of Flowers Growth Curve Grunn, the Lonely King Hound of Griselbrand Hulk, Strongest There Is Hydra's Growth Invigorating Surge Kalonian Hydra Kinetic Ooze Knighted Myr Level Up Lightning Runner Lily Bowen, Raging Grandma Mossborn Hydra Nick Fury, Agent of S.H.I.E.L.D
That includes stuff like Master Transmuter, Scarecrone, and Kuldotha Forgemaster
