This is aura Bonsai (my inner demon/brother) And the Soul Of Lumina
Umbral killed aura when he fell into the underground and gave asriel his soul when asriel obtained the soul aura's mind,personality,sprite went to umbral. whenever there's a complete genocide aura will be there and he will kill you. he will also come back if Umbral were to die,umbral were to go insane,lunar ecliptic or umbral were to be scared. umbral will try to fight aura off if its a defense mechanism or Lunar eclipse. he also has a weird obsession of carving people new faces
(side note AURA's NAME ISNT CHARA WITHOUT THE C OR THE H IT MEANS LIGHT IN LATTIN GD)