Midio is just booming with references to things I love!
First off is his name.
Midio = Midi (file format) + Mirio (My hero academia character) + Dio (KONO DIO DA)
Blûvox = Blue Vox (A parody of a band I like called Red Vox)
His hoodie being green is a nod to Ralsei (Deltarune)'s cloak thing.
The heart on the middle of his hoodie is also a reference to Spinel from Steven Universe, aka my favorite character from there.
The moon symbol on his hands is a weird obscure reference to the Pokémon Deoxys, which gets similarly shaped parts on their hands.
The eye symbol on their undershirt is a little nod to Bill Cipher.
His backstory about him being a lab creation is a reference to an Undertale fan-comic called "Handplates", where Sans and Papyrus are the lab creations of Dr. Gaster.
Midio's in-lore creator, Rysal Bortsov, is even inspired by that same Dr. Gaster himself!
Midio's single horn is a reference to the fact that I'm a fan of demonic aesthetics as well. Surprisingly, it's NOT a reference to Eri from My Hero Academia, as that's not what I intended when designing it.