defender

Fighter Archetype: Defender
Threatening: When you choose this archetype at level 3, you can threaten a target such that they focus on you.  When you damage a target with a melee attack, you can use a bonus action to mark that target; they have disadvantage on all attacks that do not include you as a target made before the beginning of your next turn.  You may mark only a single target at a time.

Shield Expertise: At 7th level, your AC improves by +1 while you are wielding a shield.  If you are adjacent to an ally, you can instead improve their AC by +1; this does not require an action, and persists for as long as they are adjacent to you.

In Harm's Way: At 10th level, if an ally adjacent to you becomes the target of an attack, you can use your reaction to swap places with that ally.  You move into your ally's space, they move into any space adjacent to you, and the attack now targets you (If the attack is an area of effect, your ally is immune if you end your turn between them and the origin point of the effect.)  This movement never provokes oppotunity attacks.

Always Ready: At 15th level, you gain a second reaction.

Intervention: At 18th level, you can rush to the aid of an ally.  If an ally within your line of sight becomes the target of an attack, you can use your reaction to move up to twice your speed into their space and move them into any space adjacent to your new position (this otherwise follows the rules for In Harm's Way, including not provoking opportunity attacks.)  If you cannot reach your ally (or if your movement is stopped prematurely), they do not move and your reaction is expended.