Copycat

A copycat can replicate the behavior and abilities they see others use. By mimicking actions, you can achieve things you normally wouldn't know how to do.

At first you can copy a d20 roll of a creature, that you witnessed, as your own, if you do something similar. Later that might be a monk's stunning strike, a fighter's maneuver or even a druid's cantrip.

Higher level spells, innate racial abilities (like a dragonborn's elemental breath) or god/patron related powers (like divine sense or pact boons) are out of your reach. If a feature is not explicitly mentioned as allowed or forbidden, the DM should make the call. Especially with other homebrew classes and archetypes, it is recommended to speak to your DM in advance.

 This archetype requires the player to be attentive and aware of what other players do and have available. As such, it is a very involved archetype, but can be very rewarding and provide some unique solutions, due to its versatility.

Copy

Starting at 3rd level, you learn to copy other creatures.

This is the basic feature of this archetype. The other features of this archetype give options of how to spend your uses of this feature.
The behavior you want to copy has to be witnessed by you (seen, heard) and taken place at most 6 seconds in the past (DM's discretion) or since the end of your last turn, if in initiative order and can be almost simultaneous (like reacting to the same trap with a saving throw).
If the behavior requires an action, bonus action, reaction or movement, you have to use the same.
You can spend multiple uses on your turn, but can't copy the same thing twice (for example only one time the fighter's Action Surge, but then something else to use this second action).

It lies in the nature of this archetype, that it is impossible to foresee any and all circumstances that might induce doubt in if and how a feature is able to be copied. So the DM has to make the call.

You can use this feature 3 times. To use it again you must finish a long rest.
The number increases by one at 9th, 13th and 17th level.

Mirror

Starting at 3rd level, you can mirror the movement and behavior of others to adopt fighting prowess, skills and avoidance tactics.

If you have to make a d20 roll for an attack, check or saving throw, you can forgo your roll and use the roll of a creature you have seen making that roll in the last 6 seconds or since the end of your last turn (It can be for the same incident, like dodging the same trap).
The roll must be of the same type (e.g. a melee attack, a DEX saving throw or a stealth check).

To use this feature you spend one of your available copy usages.

 

Adoptive Muscle Memory

Starting at 9th level, you can copy more advanced behavior of creatures.
You can use a (base) class feature of up to level 10 (no spells) of another creature, that it has used in the last 6 seconds. To do so you must use the same type of action (main, bonus, reaction).
You use the same parameters like ability modifier, range etc. but plug in your own values. Every named feature requires a separate spending of a use. As an example, you could copy a monk's flurry of blows, to make two unarmed attacks as a bonus action after taking the attack action. But in order to make it worthwhile, you would have to spend a second use to be able to roll the monk die and use Dex for the unarmed attacks.

If the copied feature is originally permanent or has a duration, the duration for you is at most to the end of your next turn (6 seconds). For example, if your fellow monk resisted a poison attack, due to Purity of Body, you can copy this and become immune to poison and disease until the end of your next turn.
If the feature scales with class level, you use the creatures class level divided by two (rounded down). 

You can *not* copy the following:
- Innate racial abilities, like the Dragonborn's elemental breath or a Genasi's resistance or features granted by items.
- Any feature with the word "divine" or "divinity" in its name.
- Any feature that triggers, when dropping to 0 HP.
- Any feature that needs more than 6 seconds to fulfill, like the Artificer's Right Tool For The Job.
- The Artificer's Infusions, the Barbarian's Rage, the Druid's Wildshape, the Paladin's Lay on Hands, the Warlock's Pact Boons, the Sourcerer's 

If in any doubt, if and how a feature is able to be copied, the DM makes the call.
Also, feel free to ask questions in the comments.

Arcane Replication

Starting at 13th level, you can copy simple magic.
You can copy a cantrip, that was cast in the last 6 seconds. If the cantrip requires a material component, you either have to have it explicitly on you or you have to be within 5ft of the original caster. If the cantrip requires concentration, you have to concentrate as per rules. If the spell scales with level, you use your level.
You can spend one usage to cast a cantrip and spend another usage to copy a sorcerer's Metamagic to improve the spell.
Your spellcasting ability is the same as the original caster's, but you use your value of modifier.

Improved Arcane Replication

Starting at 17th level, you can copy more difficult magic.

You can copy level 1 spells. If the spell needs any material component, you either have to have it explicitly on you or you have to be within 5ft of the original caster. Material components with a value have to be in your possession.