What's your language's equivalent of "John/Jane Smith" or "John/Jane Doe" — placeholder names"?

Bonus points if it's one that a person could plausibly have in real life, like "John Smith". "John Doe" and "Joe Bloggs", while common placeholder names, are unlikely to be encountered in real life — "Doe" and "Bloggs" aren't exactly common surnames in the Anglosphere.

In Vietnamese, the common placeholder male name is "Nguyễn Văn A", and the common placeholder female name is "Trần Thị B". Both employ common family names (the two most common ones), but the "first names" are just letters and unlikely to be encountered in real life. We don't really have "realistic" placeholder names I know of...