Agatha All Along fans have a new theory about Billy that shakes up what those "black heart" references could mean

Thanks to a line in Agatha All Along's opening episode, we'd all assumed that the 'black heart' that marked the final name in Lilia's list of potential coven candidates had been Rio. Now, though, it seems like that could've been a misdirect – and fans think they've worked out who it was actually referring to instead.

Warning! The rest of this article contains spoilers for Agatha All Along episode 6. If you've yet to tune in, and don't want to know anything that happens, turn back now!

The new installment, titled 'Familiar by Thy Side', takes us back in time, to before Agatha and co started walking the Witches' Road. It explains how William Kaplan, a regular kid from Eastview, came to be possessed by the spirit of Wanda and Vision's son Billy Maximoff, and that he's using the Road to find his brother, Tommy. 

Latest Videos From

Watch full video here:

In his search, Billy (Joe Locke) reaches out to Ralph Bohner (Evan Peters), a former resident of Westview who's haunted by the way Wanda and Agatha took over his life and organizes to meet him, which results in Billy learning his twin's name. Later, Billy's boyfriend Eddie (Miles Gutierrez-Riley) messages him, spiraling as to how "crazy" their night was, to which he replies: "So insane. Thanks for protecting me."

"You're my 🖤", Eddie writes back, and it's obviously not a coincidence that the youngster using the black version of the emoji...

Now, if you cast your minds back to episode 1, you'll remember that Rio (Aubrey Plaza) responded to Agatha's claim she doesn't have a heart with, "I do. It's black and it beats for you." In the second episode, then, when Lilia (Patti LuPone) gave Agatha (Kathryn Hahn) and Billy, known then as 'Teen', a list of the witches best suited to join them on the Road, the former guessed it had meant Rio – as did we. 

"That's because you used a black pen!" an annoyed Agatha exclaimed, before marching onto the streets of Westview and inviting 'Mrs. Hart' (Debra Jo Rupp) to their "party". 

Taking to Twitter to react to the new potential twist, one shocked fan tweeted: "Thinking about the fact that the black heart from ep 2 was most likely for Billy (his name probably wasn't there bc of the sigil) and not Rio..."

"So Billy has been the black heart Lilia referred to this whole time? NOT RIO," said another, while a third said: "Guys so he was the black heart? It makes sense since she couldn't write his name. Hence he's part of the coven and we're going to see his trial."

Not everyone is keen on the theory, though, whether it turns out to be accurate or not.

"The black heart was for Billy." They needed a green witch. Last time I checked Billy isnt a green witch but okay," argued one viewer, as another wrote: "The black heart thing with Billy is just a red herring to make us think otherwise about Rio. [Trust] me when I say this."

"NAHHHH IF BILLY IS BLACK HEART I WILL ACTUALLY [REDACTED]," tweeted a third.

The idea that Billy was the missing piece of the coven, and not Rio, kind of checks out, too, when you notice that he had a connection to each of the other witches prior to even meeting Agatha. Lilia read his fortune at his bar mitsvah party, three years prior to the events of the show, Alice (Ali Ahn) was one of the police officers his father flagged down when he was killed in their car crash, and he was obsessed with a skincare-based YouTube channel run by Jen (Sasheer Zamata). 

Agatha All Along episodes 1 to 6 are streaming now. Ensure you don't miss a thing with our Agatha All Along release schedule. 

For more on the wider MCU, check out our guide to all of the upcoming Marvel movies and shows or get up to speed with our breakdown of the Marvel timeline.