A 2016 Ultra Jump interview details the process that went into deciding which characters to feature in the game. The developer claims that there were 120 characters put up for consideration and that the list was gradually shortened to 80, then finally 52.

Leone Abacchio made it up to testing, but they had problems getting Moody Blues to work with the game. Since Abacchio can't really fight in a straightforward way, the developers had to come up with a system where he would replay things that had happened in the stage and thus rely completely on his opponent's power to battle. However, he was eventually dropped because the player couldn't control him precisely.

Besides Abacchio, several others nearly made the cut including Ringo Roadagain, Tarkus, Bruford, "people from Morioh", and "a bunch of others".
Contributed by ZpaceJ0ck0
Attachment
Attachment