You can see a list of default dueling spirits under dueling/spirits/default_spirits.lua. If you want to create your own custom dueling spirits, it is advisable to do this in a new file under the /spirits/ directory.

wOS.ALCS.Dueling:RegisterSpirit({
    Name = "Soul of Lexia Z'oe", -- The name of the dueling spirit
    Description = "Once a noble fighter, the wisdom of Lexia Z'oe lives on and guides you towards greatness", -- The description of the dueling spirit
    RarityName = "Legendary", -- The spirit's rarity name
    RarityColor = Color( 150, 0, 250 ), -- The RGB color of the rarity
    SpiritModel = "models/spirits/lexia_zoe", -- Model path of the spirit
    DuelTitle = "Awakened", -- The title when player enters a duel
    TagLine = "Enlightened by the past", -- The tagline when player enters a duel
    Sequence = "Animation of the dueling spirit",
    ChallengeSound = "sounds/dueling/lexia_zoe_challenge",  -- The sound that is played when player initiates a duel.
    VictorySound = "sounds/dueling/lexia_zoe_victory", -- The sound that is played when player wins a duel.
    MaxEnergy = 1000,   -- The amount of energy needed to ascend this spirit
    StartingRoll = 25, -- The base number an artifact roll will start from
    PassiveLevel = 30, -- Level of spirit before the function effects apply (OnSpawn, OnThink, etc.)
    Rarity = 25, -- Rarity of Spirit
    DroppableArtifacts = {
        [ "Sigma's Superiority" ] = 100, -- Name of the artifact, and the number required to roll 
    },
    OnDuelStart = function( ply ) end, -- Function to run when a duel has started.
    OnSpawn = function( ply ) end, -- Function to run when player spawns
    OnThink = function( ply ) end, -- Function to run each server tick
    OnDeath = function( ply ) end, -- Function to run when player dies
})