Deception +6, Insight +3

Charmed

They can Speak Common

The doppelganger can use its action to polymorph into a Small or Medium humanoid it has seen, or back into its true form. Its statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

 The doppelganger has advantage on attack rolls against any creature it has surprised.

If the doppelganger surprises a creature and hits it with an attack during the first round of combat, the target takes an extra 10 (3d6) damage from the attack.

The Actions are Multiattack, Slam, And Read Thoughts.

You would Be around To 1,000,000,000 to 900,000,000,000 Years old

 Darkvision 60 ft

The Creature Has adveantedjeve While being in The UNDERDARK, And Urban areas.

Doppelganger can live to be more than a unknown of years of life, so the Doppelganger often remember a very different world. For example, some of the Doppelganger living in Citadel Felbarr (in the world of the Forgotten Realms) can recall the day, more than three centuries ago, when orcs conquered the fortress and drove them into an exile that lasted over 250 years. This longevity grants them a perspective on the world that shorter-lived races such as humans and halflings lack.

You gain proficiency in the Intimidation skill.

When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead. You can’t use this feature again until you finish a long rest.

When you score a critical hit with a melee weapon attack, you can roll one of the weapon’s damage dice one additional time and add it to the extra damage of the critical hit.

You have proficiency in the Athletics skill.

You can focus yourself to occasionally shrug off injury. When you take damage, you can use your reaction to roll a d12. Add your Constitution modifier to the number rolled and reduce the damage by that total. After you use this trait, you can’t use it again until you finish a short or long rest.

You count as one size larger when determining your carrying capacity and the weight you can push, drag, or lift.