As you were originally a Human, you have the same size and build, so your size is Medium.

Your walk speed is 30ft

Being a Demi-fiend, your power comes from ingesting the demonic Magatamas. Strange worm-like creatures that can be changed for different abilities. Once per Long Rest, you can alter your resistances to one of the following:

Marogareh: AC 10, No Immunities/Resistances. No Vulnerabilities. You get +1 to Attack/Hit when using an unarmed attack or an attack with a melee weapon. At Level 12, your melee weapon and unarmed attacks are treated as Magic damage unless the enemy is immune to Slashing, Bludgeoning and/or Piercing damage.

Shiranui: AC 12. Immune to Fire. No Resistances. Vulnerable to Cold.

Wadatsumi: AC 12. Immune to Cold. No Resistances. Vulnerable to Lightning.

Narukami: AC 12. Immune to Lightning. No Resistances. Vulnerable to Fire.

If you use any Armor while having ingested Shiranui, Wadatsumi or Narukami, the natural AC becomes 11 and Immunities turn into Resistances..

While in Darkness or Magical Darkness, the tattoos glow on your body, giving you 20ft of dim light. Color changes between two of your choosing. One while above half-health, and one while below half-health.

The power you wish for yourself is seemingly up to you, to shape it how you see fit. You gain +2 to your Constitution score. In addition, you may choose one of the following:
+1 to your Strength Score
+1 to your Intelligence Score
+1 to your Charisma Score

Your connection to the Demonic has granted you to know things you were not previously privy to, as if it was being whispered in your ear. You gain proficiency to Insight and Arcana checks.

The spike at the back of your neck seems to be connected to your Demonic self, giving you the ability to understand and communicate with both Angels and Demons. You can read, speak, and write Common, Celestial, Abyssal and Infernal.

When being identified though Divine Sense, you will always read as a Demon, exposing your dark nature.

Being chosen to fulfill some sort of world-changing destiny, they tend to be rather Lawful and faithful to the path they choose and will follow it through to the end, whether it be for good or ill. It can be persuaded, though extremely difficult to do so, and will plot to eliminate anyone or anything that stands against its Reason.