Racial Variants (Avatar Supplement)
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All of the characters in the Avatar: The Last Airbender setting are human, but their country of origin has a major influence on their abilities, especially if they have the ability to bend one of the elements. The four "races," then, in the campaign setting here would be the societal differences between the four nations: Air Nomads, Earth Kingdom, Fire Nation, and Water Tribe. In other settings or with mixes, I recommend trying out different homebrew races or the classic 5e races. Have fun with it!
A historical note on racial variants: at the time of Avatar Korra, the four nations had become more unified, especially in Republic City. Families blended between nations, and bending was most often based on parentage. For example, the daughter of an Earth Kingdom/Fire Nation union could have the abilities of a firebender, an earthbender, or a nonbender!
Also, following the Harmonic Convergence, many former nonbenders from all four nations suddenly gained the ability to airbend.
There are many ways to go with the racial variants, but here are my suggestions.
General Human Traits
Player's Handbook pg. 31
Age. Humans reach adulthood in their late teens and live less than a century.
Alignment. Like the typical 5e human, alignment varies greatly depending on background, personality, and motivation. Every human in this setting is capable of great good and great evil.
Size. The average human stands at 5 to 6 feet tall, weight varying greatly. Size is Medium.
Speed. Base walking speed is 30 feet.
Languages. You can speak, read, and write Common. In the Avatar setting, Common is the only language spoken.
Alignment. Air nomads tend toward the True Neutral, based on their spiritual bent. Due to their value of freedom, they may also tend toward Chaos.
Evasive. Proficiency in the Acrobatics skill.
Alignment. Earth Kingdom citizens are as varied as it comes.
Stubborn. Proficiency in the Persuasion skill.
Alignment. Fire nation citizens tend toward Law.
Determined. Proficiency in the Intimidation skill.
Alignment. Water Tribe citizens are as changeable as their element, but often tend towards the Neutral, acting the mediator between Law and Chaos.
Empathetic. Proficiency in the perception skill.
Republic City (Post-Aang)
Alignment. Citizens of Republic City can be the best, the worst, and everything in-between.
Educated. Proficiency in the History skill.