This is generally handled on a case-by-case basis depending on the character(s), but in this case, both Eddie and his symbiote are solid enough characters on their own and should be apped separately.

As for Eddie reacts without his symbiote, that is completely up to how you wish to play him. However, the symbiote would NOT be inside of him. As for the symbiote, if it does not have a canonical human form, you would be free to come up with one. It could look like Eddie if you wish, but it could not be a pitch black humanoid, as all characters are humanized upon arrival to Ryslig.

As for merging two characters together, we're afraid this wouldn't be permitted longterm. However, you're free to purchase the ability to temporarily merge with another character with the coins you earn through activity. Once you're in the game proper, we can discuss the cost!