Your Constitution score increases by 2.

Koopas attain a similar size and development as humans, and reach adulthood by 18. They can live to be around 300.

Koopa tend to extremes, making a conscious choice for one side or the other in the war between the Mushroom Kingdom and Bowser's Kingdom. Most Koopas are neutral, but those who side with Bowser can be villains.

Koopas have a variety of sizes and shapes, standing well over 4 feet tall and averaging almost 150 pounds. Your size is Medium.

Your base walking speed is 30 feet.

You can use your action to launch yourself at multiple enemies in a chain if each is within 10 feet of each other. Ignoring terrain you'll essentially glide to each creature which they must make a saving throw. The DC for this saving throw equals 3 + your Dexterity modifier. A creature takes 2d4 damage on a failed save, and no damage on a successful one. The damage increases to 3d4 at 6th level, 4d4 at 11th level, and 5d4 at 16th level. You can only attack an enemy once when attacking  each enemy. You'll land within 10ft of your last target.

You can use your action to hide your body within your shell.  When you use Shell Shield when attacked you must make a saving throw. The DC of this saving throw is 10. You take half damage on a failed save, and no damage on a successful one. After you use Shell Shield on a turn you are immobile and cannot use your reaction until your next turn.

You have resistance to piercing damage.

You can speak, read, and write Common.

There are many subraces of koopa troopas. Each has its own advantages and disadvantages + the benefits of being part of the koopa race.

Hammer Bro (Variant)

As a hammer bro, you have a strong bond with the team, your keen intuition and your strength allow you to overwhelm enemies that get in your group's way.

Ability Score Increase
Your Strength score increases by 1.

Bros. Training
You have proficiency with hammers and hammer-like weapons.

Fleet of Foot
Your base walking speed increases to 35 feet.

Cruel Soloist
When you're engaged in combat and a companion isn't within 50 feet of you, role at disadvantage for every attack.

Magikoopa (Variant)

Koopas who were raised and learned to spell cast. Often they are considered high ranking in Bowser's army.

Ability Score Increase
Your Wisdom score increases by 1.

Polymorph
You can cast the polymorph spell once using this trait. You regain the ability to do so when you finish a long rest.

Flight
You have a flying speed of 50 feet. To use this speed, you can’t be wearing medium or heavy armor.

Fragile
When it comes to Constitution saving throws, roll at disadvantage.

Dry Bones (Variant)

When a koopa perishes sometimes they rise from the dead with some of their intelligence intact.

Ability Score Increase
Your Dexterity score increases by 1.

Darkvision
Accustomed to dark castles and haunted mansions, you have superior vision in dark and dim conditions. You can see in dim light within 60 feet of you as if it were bright light, and in darkness as if it were dim light. You can’t discern color in darkness, only shades of gray.

Poison Immunity
You are immune to poison damage. You do not require  air,  food, or drink.

Undead Nature
Your creature type is undead.

Paratroopa (Variant)

When koopas have wings they become formidable enemies to land dwellers, but a paratroopa's wings are fragile.

Ability Score Increase
Your Dexterity score increases by 1.

Flight
You have a flying speed of 50 feet. To use this speed, you can’t be wearing medium or heavy armor.

Plummet
When you're attacked in the air, make a Dexterity saving throw of 12 or more. If succeeded you do not fall down, if you fail you plummet to the ground taking fall damage.