V8P3 39%:
Was he one of the Louis's...?
The apostrophe makes it possessive, not plural. It's probably better to go with Louises for plural (even though it looks the same as the plural for Louise). Unless there's a different rule for plural proper nouns in French? https://www.merriam-webster.com/grammar/what-happens-to-names-when-we-make-them-plural-or-possessive