The Sentinel

There are many reasons that one would fight. Freedom, power, money, or fame. Sentinel's are a different breed, fighting to protect others. They are watchful guardians, moving in to defend and guard individuals from threats much greater than them. They are towering warriors, with broad shields and thick armour that few can match.

Guardian's Ward

When you choose this archetype at 3rd level, as a bonus action, you can designate an ally you can see as your ward.(Can't be yourself) When you declare a creature as your ward, you can move up to half your speed towards the ally, without spending any movement. You must end this move closer to the ward than you started. The target remains your ward until you use this bonus action to target a different creature, you end the ward on your turn (no action required), you die, or your ward dies. You gain the following benefits while you have a ward:

• While you are within 5 feet of your ward, your ward gains the benefits of half cover. (At 15th level, this benefit increases to three-quarters cover.)
• You have advantage on ability checks made to spot and prevent threats to your ward. 
• When you and your ward roll initiative, you can choose to take your ward's initiative roll instead of your own. Your ward decides who gets to act first.

Shared Experience

Also at 3rd level, you've been trained to assist your ward and anticipate their needs. If your ward makes a skill or tool check while they are within 15 feet of you, you may use your reaction to allow them to add your aligned ability score bonus to their check. You may use this ability a number of times up to your proficiency modifier and cannot use it again until you have finished a short or long rest.

Watchful Sentry

At 7th level, when you are within 5 feet of your ward, and a creature you can see enters within 5 feet of your ward, you can use your reaction to swap places with your ward and make an opportunity attack against that creature, interrupting their turn.(You don't skip their turn, they continue it after you use this ability) This movement doesn't provoke opportunity attacks. Furthermore, creatures provoke an opportunity attack from you when they enter within 5 feet of you.

At 13th level, you can also use your reaction to force ranged attacks that target your ward to target you instead until the end of your ward's next turn.

Synergy with the Protected

Beginning at 10th level, you have advantage on any attacks you make against creatures that your ward has damaged since the end of your last turn. (0 damage is still considered damage for the purposes of using this ability). Additionally, if you hit a creature that has made an attack against your ward since the end of your last turn, you can add additional damage equal to your fighter level to the attack. You can do so no more than once per turn. 

Defender's Vow

Starting at 15th level, as a bonus action, you can position yourself in front of your ward and brace incoming strikes. You gain temporary hit points equal to twice your Fighter level, which last until the beginning of your next turn. While these temporary hit points persist, your ward gains the benefits of total cover while they remain within 5 feet of you. You can use this feature a number of times equal to your Constitution modifier (a minimum of once). You regain any expended uses when you finish a short or long rest.

Army of Two

Finally, at 18th level, when you and your ward make a Strength, Dexterity, or Constitution ability check or saving throw against the same effect while within 5 feet of each other, you can substitute your ward's d20 roll for your own. You can wait until after both of you roll the d20 before deciding to use this feature, but you must decide before the DM says whether the roll succeeds or fails. Additionally, your ward gains a bonus to their AC & saving throws equal to half your proficiency bonus (Rounded up) if they remain within 30ft of you, and you can reach them with your movement speed.