While many fighters study the blade or bow, Guardian fighters study the shield. Not only are you better at handling a shield to protect yourself, but your skill in protection lets you help your allies as well.

Level 3: Greatly armored

Your study of self-protection aids you in battle. You gain the defense fighting style, if you already have this fighting style, chose a different one.

Shield Smasher

At level three, your defense can be your offense. You can use (if wielded) a shield as a weapon. It will have a base of 1d6 + STR damage. If the shield is a magic, you will add the shield's bonus to AC to the attack roll and damage value. This weapon can be used on a bonus action as well as an attack action.

Protective stance

at 7th level, you are able to aid allies within 15ft of you in the following ways:

You can use your reaction when you see or hear an ally that is within 15ft of you take damage to take that damage onto yourself instead. You take this damage without any resistances or immunities affecting the value. This does not prevent or transfer any spell effects besides damage.

When an ally rolls an ability check within 15ft of you, you can use the class ability Tactical Mind for their roll. 

Weakening strike

At 7th level, you can once per turn attempt to reduce your enemy's armor class until the start of your next turn. You can attempt this after successfully landing an attack against a foe. The enemy then has to roll a DEX save against a DC = Your AC. If failed, the enemy's armor class has your PB subtracted from its AC until the start of your next turn. 

Unarmored enemies roll this save with advantage.

Quick Breather

at 10th level, you gain the following:

You roll 2d10 when using a second wind and chose the better result.
You gain a second wind after killing or knocking a foe out.

When taking a short rest, you can roll one extra hit dice, that dice's value + your CON modifier are added to all the other member's of the party's HP as either normal HP or temp HP (the ally's choice)

You gain a heroic inspiration when an enemy rolls a natural one against your AC

"Get Behind Me!"

At 15th level, your protection of others allows your allies to be protected inside and outside of combat:

When an ally is standing 5ft behind you and both of you are targeted by an area of affect like fireball, a dragon's breathe weapon, and etc. the ally rolls any save needed due to that effect with advantage and suffers no damage on a success.
When standing 5ft behind you, an ally is given half cover, unless they have taken the stealth action, where they instead gain full cover. This cover cannot hinder the ally's attacks, only an enemy's.

When using a second wind for either a group skill check or to heal yourself during combat, any ally within 15ft of you also gains the bonuses you do.

Empathic guard

At 18th level, You gain the ability to assist allies under spell and natural effects that influence the mind. When an ally has failed a WIS, CHA, or, INT saving throw and is put under an effect, you can use a bonus action to remove the effect, curing them from the affect. You can only do this bonus action once per short rest.