Pretty sure it's just referencing the Day of the Dead.
Actually, as the commentary says, this is a reference to La Santa Muerte... I could try to explain what that is but the link provided in the commentary explains it well.
I know Mefo was thinking in some mesoamerican god but kinda remebers me of Hel, half alive half corpse
Fun fact, the souls of the dead were believed to take the form of monarch butterflies, so a depiction of the Day of the Dead would probably look a lot like one of Yuyuko's spellcards.
