Companion Fighter

Some fighters, rather than focusing on increasing their own martial prowess increase their fighting capabilities by teamwork with an animal companion. 

Companion

At 3rd level, you gain a beast companion that accompanies you and fights alongside you. Choose a statblock for a beast, no larger than Medium and that has a challenge rating of 1/4 or lower. Those are the stats for your companion. The companion does not need to be the same beast as the statblock, but is up to the PC and GM to decide a fitting companion, I.e if the GM allows it, the companion could be a medium sized brown bear with the stats of a Wolf. Add your proficiency bonus to the beast’s AC, attack rolls, and damage rolls, as well as to any saving throws and skills it is proficient in.

Its starting hit point maximum is 4 times your fighter level and adds constitution to its hit point maximum as if it were the same level as your fighter level.

The beast obeys your commands as best as it can. It fights on the same initiative as you, but you can choose in what order you fight. On it's turn it moves as normal, which requires no action by you. Whenever you take the  Attack, Dash, Disengage, or Help action, you can choose to have your companion instead take that action. If your companion takes no action the beast instead takes the dodge action, no action required. Once you have the Extra Attack feature, you can freely split those attacks between you and your companion.

If you are incapacitated or absent, the beast acts on its own, focusing on protecting you and itself. The beast never requires your command to use its reaction, such as when making an opportunity attack.

Your companion and you share the hit  dice pool. Whenever you spend hit dice to heal yourself, the companion heals for the amount rolled, plus its constitution modifier.

The Companion and you can understand each other well, by reading the companions motions and sounds, you can understand simple ideas and how it's feeling. The companion can understand one language, which you know.

If the companion drops to 0 hit points, it takes death saves as if it was a PC. If the beast dies, you can bond to a new companion. It takes 8 hours to bond with a beast which you are friendly with and meets the requirements.

Safe Rests

Whenever you take a long rest and spend the whole rest with your companion, sleeping together and, or keeping watch together, you regain all of your spent hit dice.

Additionally, whenever you use your second wind bonus action, the Companion can also regain the same amount of hit points, however, it may not take any actions during that turn.

 Starting at level 7, when you spend a short rest using no hit dice to recover hit points, you regain half of your hit dice, if you spent the rest with your companion. This feature can only occur once per long rest.

Teamwork

At 7th level, you have learned to work together to gain even greater efficiency in battle. During your turn, you can spend one hit dice to activate one of the following tactics. You can spend multiple hit dice on the same turn, but you can't activate the same tactic twice during a single turn.

• Protective. Nominate one ally, whenever an enemy attacks this ally, they have disadvantage on the attack as long as your companion is within 5ft of the ally. This effect lasts until the start of your next turn.

• Swift. You and your companion gain +10ft movement. Additionally you and the companion can dash as part of your bonus action. This effect lasts until the start of your next turn.

• Unpredictable. You and your companion trigger no reactions during this turn. (sentinel)

• Double Team. If you attack a creature or object which your companion is within 5ft of, all attacks by you and the companion are made with advantage until end of turn.

• Charging. When your companion moves 15 ft before attacking a target, they deal critical damage with every hit.

• Territorial. Other creatures provoke an opportunity attack from you and your companion when they enter your reach. Your companion does not need to use their reaction for these attack, and could therefore attack multiple incoming creatures.

• Surge. Your companion gains an action. If this action is used to make the attack action, then it can't be given any attacks by your attack action, the companion does gain the benefit of your extra attack feature.

• Savior, If your companion is within 5ft of you and you are incapacitated, it stabilizes you.

Savage Strikes

At the 7th level, your companion's attacks count as magical to overcome resistance.

Companion Feat

At 10th level, your companion gets to choose a feat from the companion feat list.

https://www.dndbeyond.com/feats/730710-companion-feats

Companion Archetype

At 15th level, you choose one of two features, you gain an additional feature depending on your choice at level 18.

 

Defensive

Gain one additional use of your second wind feature. Additionally, when you use second wind, you regain one spent hit dice.

Offensive

When you use your action surge feature, both you and your companion gain an additional action. When you use your action surge ability, you may also activate one tactic without expending a hit dice.

A fighters best friend

At 18th level, you gain a feature depending on what you chose at level 15.

Defensive

Each time you expend a hit dice on a tactic, you may also roll that dice and heal your companion for that amount of hit points.

Offensive

Each time you expend a hit dice on the Teamwork feature, add that dice to a pool. When your companion hits with a weapon attack, you may expend any of those dice to add them to the damage of the attack. This pool of dice lasts until you gain hit dice, either from a long rest or from a short rest with the safe rest feature.

These additional dice only double from a critical hit if the dice rolled a natural 20. It will not double as a result of the charging tactic.