Untested, but I think this is roughly how you'd want to set it up, if I've not made any mistakes*
Code:
custom_columns_settings:
characterlist=>#yourcharactercolumn
replace_metadata:
## Fix and standardize your character names first
characters=>Harry P\.=>Harry Potter
characters=>N\. Tonks=>Nymphadora Tonks
## If you'd like, any characters not listed here can be grouped under "minor characters"
characterlist=>^(?!.*(Harry Potter|Nymphadora Tonks)).*$=>Minor Characters
## Then use a conditional to prefix the fandom - you'll need to do each one individually
## It may also cause weirdness with crossovers
characterlist=>^(.*)$=>Harry Potter.\1&&category==(Harry Potter)
# 'add_to_' concats on to existing param, thus the leading comma
add_to_extra_valid_entries:,characterlist
#characterlist is a copy category for Fandom.character
include_in_characterlist:characters
*to Jim: apologies in advance if I get this wrong and you need to correct it
There might be a better way to handle the conditional but I'm not sure how do to it.