Green Knight's specialties include Poison DoT (3x higher damage than Fire DoT potentially), crowd clearing, basic juggling, and basic boss slaying.
Green Knight's element is Poison. All poison attacks do three hits of DoT. However, some enemies such as Cult Minions, are highly resistant to Poison and are immune to its secondary effects. Which makes it about as effective as Fire and is a relatively good element.
Damage/Hit: Base Magic Damage x 0.5 + Poison DoT x 3
A poisonous cloud pillows forth, knocking over any enemies it hits and poisons them. The poisoning damage over time effect makes this pretty powerful and potentially superior to fire. Each upgrade level adds to the range and to how many max hits the spell can inflict on an enemy that is not knocked over by it. This spell's collision is a bit odd in that it can go through a decent amount past terrain obstacles such as walls, pits, and the line of the volleyball game, albeit it won't fully go past the line unlike Industrialist and Fencer's splash attacks, which fully can.
A ball of poison. The Poison DoT is rather nice in killing enemies quicker; more so than Fire DoT. Since Green Knight has a different YY/△△/Whatever key you have binded in place of YY) than all of the other 30 characters in the game, assuming you know how to levitate (XYY/□△△/Whatever you key you have binded in place of XYY) Level 8 required) + spellweave (RT+Y/RT+△/Whatever key you have binded in place for RT+Y) 10 points allocated into Magic), Green Knight can cast 5 slanted Magic Projectiles while still in the air, unless you also know how to Slow Levitate (RT+A AA XYY/RT+X XX □△△/Whatever key you have binded in place for RT+A AA XYY) in which you can cast slanted Magic Projectiles infinitely and easier.