Sage Ranger
This is the most niche unit in the deck. Its obvious main use is poke at key targets, especially the enemy summoner, but also some low life units hiding behind others. It has particularly good range not only because of its shoot-through ability but also its ability to move further on the turn summoned, sometimes allowing you to bring Shadow Knights to the frontlines more readily and unexpectedly.
But for the most part, Sage Ranger should be used sparingly with the other 3 commons fulfilling the key roles.