has.YourCostume = false
Mod.JULIEN = Isaac.GetCostumeIdByPath("gfx/characters/julien.anm2")

function Mod:onInit(player)
    if game:GetFrameCount() == 1 then
        if player:GetName() == "Julien"  -- not sure about the character
        and has.YourCostume == false  then
           has.YourCostume = true
           player:AddNullCostume(Mod.JULIEN)
        end
    end
end
Mod:AddCallback(ModCallbacks.MC_POST_PEFFECT_UPDATE, Mod.onInit)