However, on chambers online it is referenced only as "case = an odd character" as the 8th possible meaning of the 2nd entry.
The word eccentric refers to people who are only a subset of people who are odd characters: I would say that is a pretty vague link. I didnt say invalid, just vague enough for it to "rather spoil" the clue for me.
As an aside, I personally don't love solutions that rely on crossword ready lists of synonyms; these are rather self-referencing and contribute to a feeling that insider knowledge is required to solve. The "Never would have come up with that myself but it is in my Bradford's!" solution. Just my view.
We can agree to disagree.