The Sentinel is the second sub-class for the Titan. The Void-based sub-class wields a Void shield that can be used in close quarters and as a thrown projectile. The Sentinel's super ability is the Sentinel Shield, a duel use weapon that allows for strong melee with a mixture of ranged attacks (similar to Captain America).
As you'll note below, the Sentinel Titan gains Grenades which offer utility in the form of denying ability use, or additional damage, while his passive trees provide much greater survivability (Code of the Protector) or much greater damage output through cooldown reduction (Code of the Aggressor).
Super
Sentinel Shield: Summon a shield of Void Light. While Sentinel Shield is active, you can attack or throw the shield to your foes.
Grenades
Magnetic Grenade: A grenade that attaches to enemies and explodes twice.
Suppressor Grenade: An explosive grenade that prevents enemies from using abilities for a short time.
Voidwall Grenade: A grenade that creates a horizontal wall of burning Void Light.
Barrier
Towering Barricade: A large barrier that can be used for cover, or to seal off dangerous areas for a short time.
Rally Barricade: A small barrier that allows you to peek over it while aiming down sights, and instantly reloads your equipment when you take cover.
Lift
Increased Height: Upgrades Lift to travel to greater heights.
Catapult: Upgrades Lift to provide a strong initial burst of momentum.
Increased Control: Upgrades Lift for better directional control while in the air.
Passive Tree - Code of the Aggressor
In The Trenches: Kills while surrounded by enemies reduce the cooldown of your Super.
Second Shield: Gain an additional Shield Throw charge while Sentinel Shield is active.
Shield Bash: After sprinting for a short time, use this Melee ability to unleash a devastating Shield Bash that disorients enemies.
Superior Arsenal: Grenade kills recharge your Grenade energy.
Passive Tree - Code of the Protector
Defensive Strike: Kill an enemy with this melee ability to create an overshield around you and nearby allies.
Rallying Force: Melee kills restore health for you and nearby allies.
Turn The Tide: Your Overshield from Defensive Strike lasts longer and increases melee damage and reload speed.
Ward Of Dawn: When Super energy is full, press and hold the top shoulder buttons to create a shielding dome to protect you and your allies.