I get a kick out of linguistic quirks, and this one’s neat: if you want a synonym for 'monarch' that specifically implies a ceremonial, powerless role, 'titular monarch' or 'nominal sovereign' fits best.
People toss around 'figurehead' casually, and that works in everyday speech, but it has a blunt, slightly dismissive tone. 'Titular' comes from the idea of holding a title in name only; it signals that someone is the official head but not the real seat of power. 'Nominal sovereign' says the same thing in a fancier register. 'Constitutional monarch' can also indicate a ceremonial role, but it’s broader — it refers to a monarch whose powers are constrained by a constitution and political reality, which is often ceremonial but not always.
If I’m writing something polished, I’ll choose 'titular monarch' when I want precision and a slightly formal flavor. If I’m chatting online or in a comic-style caption, 'figurehead' gives the reader an immediate image. For historical or political contexts, 'constitutional monarch' is safest when the legal framework matters. Personally, I like how 'titular' sounds a little dramatic and a little melancholy — like someone who carries a crown purely for the stories, and that always makes my imagination wander.