Pre-release WMG
Unmarked spoilers!
"The real Agent Argylle" is just Aidan the whole time
- There's a whole moral of the story going on, that basically real spy work is nowhere near as glamorous as in the novels. Instead of someone like Henry Cavill you're going to get someone like Sam Rockwell.
- Jossed. The real "Argylle" turns out to be Elly herself — real name Rachel Kylle, even if she sees Argylle in Aidan for a while (Aidan is the real life counterpart of Wyatt, Argylle's partner in the novels). But then a real guy who has the same (if way more unkempt) face as Argylle and might be a spy shows up at Elly's book presentation (he's also played by Henry Cavill).
"The real Agent Argylle" is the cat
- Don't ask me how or why, but that's my WMG. Given the stuff Matthew Vaughn is into I wouldn't put it past him.
- Jossed.
Post-release WMG
In Universe, the fans ship Agent Argylle and Wyatt.
- Since Elly/Rachel was basing the books subconsciously on her old life and those two characters represent herself and Aidan, then Argylle and Wyatt's relationship in the books likely had at least some hints of the actual romantic relationship that fans pick up on.
The man who looks like Argylle in the final scene is a Statesman agent
I mean, that accent is a dead giveaway, right?