Graphics and now videos? This has been quite entertaining x)!
Here are some ideas, use if you need em!:
How about introducing a yet to be named starving critter as a possible pet sidekick for metty? It's a white critter with red ears... You might tame it if only you had some birthday cake... Maybe it's intelligent and uses thought bubbles. Even if it can't talk. There's room for him to grow. Oh, maybe its red ears glow in the dark?
Or how about The Eye? The Eye can possess any inanimate object and do stuff. It might have an attitude after its starring role was taken by some rabid robot with a gun. It took a while for it to recover from the indignity. Surely it means no harm and doesn't hold a grudge. *Maybe* if it possessed a door it could transport someone somewhere? As long as there is a door, it could send you anywhere! If it feels like it... It can talk somehow and its name is Aiuctegh'vhash (maybe just call it Hash or something if you can't pronounce it)
Or if you really want an OC that's a person ...
I was thinking a private investigator using the shady guy from the credits (above Achoo! / below nurse hasumi)
https://www.spriters-resource.com/pc_computer/cavestory/sheet/2292/
He'd probably be willing to take on the case of the missing girl on an island... If he had a name... it would be... Jones. Mr. Jones.