Due to being a cute fluffy squirrel, you get a +4 in dexterity and a +2 in charisma, but also due to being a cute fluffy squirrel, you get a -4 in strength and a -2 in constitution.

As a squirrel you are relatively quick and can keep up with humans. You are also a natural climber from living in tree’s your entire life. You have a walking speed of 30ft and a climbing speed of 35ft

Your size can range from around 15-20 inches (38-50cm) and you weigh about 1-1.5lbs (0.45-0.68kgs). Your size is tiny.

You can jump an equal length to their movement speed from a running start and a third of that from a standing one due to having to jump from tree to tree. You can also jump vertically a quarter of your movement speed up.

After falling out of trees a few times you learnt to land on your feet. You take no fall damage from a fall less than 60ft. You can also roll a Dex save up to 90ft to take half damage.

Due to your itsy bitsy teeth you can deal 1d4 piercing damage when you bite someone. This is basically your unarmed strike because you have no hands to to whack people with. You can take this attack as a bonus action, but it only has 1ft of range so you have to basically be on top of them.

You have very good senses from living in the woods. You gain advantage on perception checks requiring hearing or smell. 

You can communicate with animals, due to you being an animal yourself.

You can ride on your allies (or enemies) shoulder, head e.t.c. Being tiny means you can fit in the same space as an ally/enemy.

Being a squirrel, you have no opposable thumbs. This means you can’t use weapons unless they are specially made for you. The base cost of any weapons is 2x the normal price. The same goes for armour. Your armour has to be specially made meaning the cost is 2x the base price. Wearing medium armour reduces your jump distance by half, and heavy armour reduces it to a quarter.

You can now speak and read common for the purpose of taunting the humans, and infernal to communicate with your devilish kin. You can write but with a lot of struggle.

When you made the deal with Satan, you agreed to send people to hell for him in exchange for some extra powers. He made you a sentient squirrel as well as gave you some extra ability’s to help in your quest. You have the fire bolt cantrip and produce flame cantrip, and they don’t count against your prepared spells. You also get the meteor swarm spell at level 15 (because it’s a 9th level spell) to use once per long rest. This also does not count against your prepared spells.

You gain advantage while hiding in trees.

You have a natural AC of 18 because you are so small and hard to hit. Shields and armour don’t affect this, though you may use that instead if it’s higher. Your hit die are permanently d4s no matter your class, however, also because of how small you are.