Many fans were surprised to learn in spring 2012 that Quicksilver and Scarlet Witch could be used by both Marvel and Fox, a scenario that materialized about a year later with Joss Whedon announcing the siblings' inclusion in The Avengers: Age of Ultron, and Bryan Singer casting Evan Peters as Pietro Maximoff in X-Men: Days of Future Past.

However, it turns out they're not the only characters living in that rights "gray area."

“There are only a handful of characters that occupy that middle ground,” Kevin Feige, Marvel Studios' president of production, tells the New York Daily News. “Iron Man is not going to show up over there and Magneto is not going to show up over here. But there are a few gray points even after many years of negotiations … and that only happens with a character like Quicksilver, who has been a part of the X-Men, the son of Magneto in those comics, but also a primary Avenger. The characters (on the screen) are going to be very different. Other than their powers, general audiences might not even know — unless they read your article — that the two are related.”

Scarlet Witch and Quicksilver, the mutant children of Magneto, have long histories with the Avengers and the X-Men: Although they were introduced in May 1964 as members of their father’s Brotherhood of Evil Mutants, they joined the ranks of the Avengers within a year. It's difficult to think of other characters who have such deep roots in two separate Marvel franchises -- and who might constitute the rest of that "handful."

Following the casting of Peters in May, a report indicated that Marvel and Fox were in a "legally negotiated stand-off," with the former prohibited from making reference to mutants or Magneto in the Avengers sequel.

(via ComicBookMovie)