Passive: While within 175 units of an allied champion, you raise your Guard. Allied champions you affect with unit-targeted abilities are also Guarded for 2.5 seconds, irrelevant of distance.
If you or a Guarded ally would take damage from an enemy champion, you all first gain a shield for 60 - 150 (based on level)(+ 20% Guardian's AP)(+ 10% Guardian's bonus health) as well as 20% bonus movement speed for 1.5 seconds.
Guardian only goes on cooldown when the shield is triggered.
The auto-targeted shield from Thresh'sDark Passage has been special-cased to apply Guard, but no other auto-targeted ability will apply the effect. Other targeting types are also excluded.