A person who knows how to change their body for the situation.

Adaptive Senses

Starting at 3rd level, during a short rest, you can choose one additional sensory.

You can also choose Tremorsense and Truesight at 7th level and Keen Senses at 11th.

Blindsight

Blindsight up to 60 ft

Darkvision

Darkvision up to 60 ft

Keen Senses

You have advantage on perception checks when using smell or hearing.

Tremorsense

Tremorsense up to 60 ft

Truesight

Truesight up to 60 ft

Adaptive Movement

Starting at 3rd level, during a short rest, you can choose one additional.

You can also choose Burrowing speed at 7th level and Flying speed at 11th.

Burrower

Burrowing speed of 30ft

Climber

Climbing speed of 30ft

Flyer

Flying speed of 30ft

Sprinter

Walking speed of 45ft

Swimmer

Swimming speed of 30ft

Adaptive Ability

At 3rd level, during a short rest, you can choose an improvement to your ability scores, checks, or saves.

Able

Two Ability Points

Aware

You can give yourself advantage for any roll a number of times based on your wisdom modifier per long rest.

Capable

You gain proficiency in your choice of saving throw.

Expert

Expertise in one Skill

Jack of All Trades

Half Proficiency on Ability Checks

Resilient

+3 on Saving Throws

Secrets of the Doppelganger

By 7th level, you have studied the secrets of the shape changing of the Doppelganger.

You cast a modified Alter Self where you can be a small to medium humanoid, but you do not get the other features of Alter Self. This is an unlimited spell.

Once per long rest, you can also change one of you adaptations as a bonus action.

Creature Studies

Starting at 11th level, you can make physical attack actions on reactions, melee or ranged against your Hunters Marked target. 

Also, if you spend a minute studying a person, you can have advantage on charisma checks when pretending to be them while using Secrets of the Doppelganger.

Adrenaline Gates

At 11th level, you learn spent enough time altering yourself to develop a fully responsive adrenal system.

As an action, you can cast the Haste spell on yourself. 

If the spell has reached the end of its duration, the player can choose to add a number of turns equal to their Constitution Modifier to it and take 1d12 damage to themselves.

You can continue to do this until you hit 0 hp. 

If you are forced to make a constitution saving throw to concentrate on Haste and fail, you can take a 1d12 and recast Haste with as man turns equal to your Constitution Modifier.