There is a dimension where Dr Gratton and Mr Umbris are a thing, I called it The Strange Case AU. It's like Cornelius is Jekyll, Deimos is Hyde
Some headcanons:

- Dr Gratton invented a formula (a potion) in hopes to separate himself from his “negative feelings”, for the sake of “becoming a decent flawless person” + to help people once he improves it via testing on himself

- Mr Umbris is a side of his personality that he struggled to suppress for his whole life, which became (partly) free after drinking the potion a few times

- Umbris’ first name is Dorian

- he is younger than Cornelius; both physically and mentally 

- his eyes are green and his hair is darker

- Dr Gratton is aware of his existence (black outs), but it is pretty much alike a case of DID, so he doesn’t have memories of what Umbris was doing while being in control

they are capable of interacting with each other in dreams and in hallucinations (due to the lack of sleep)