Marvel has many distinctive superteams including the Avengers, the X-Men, and the Guardians of the Galaxy.
Even though these groups have key members, nobody stays on forever. Wolverine and Storm may be among the most famous X-Men but that hasn't stopped them for having a few solo adventures. Hulk is synonymous with the Avengers but he quit after only two issues.
But one group that many true believers assume is set in stone is the Fantastic Four. Mister Fantastic, the Invisible Woman, Human Torch, and the Thing were the founding members and still are the official line-up, even after 60 years.
But that doesn't mean they haven't had the odd break. Every once in a while, one of them is forced to leave due to an injury, personal problems, or from suffering from a bad case of being dead. So, when one of the Marvel's First Family is out of commission, there needs to be a replacement. (I mean, you can't call them the Fantastic Three, can you? Doesn't have the same ring to it.)
So, what other superheroes have joined the Fantastic Four over the years?
12. Crystal - Fantastic Four #81-105 (1968-1970)
Crystal Amaquelin is mainly known as the princess of The Inhumans Royal Family and Johnny Storm's central love interest. However, the stripey-haired superheroine also gets credit for being the first new member of the Fantastic Four.
When Invisible Woman became pregnant, her teammates thought it sensible for her to take a sabbatical for a while. Because Crystal had the power to control the elements and had helped the Fantastic Four on several occasions, they thought she would the perfect candidate to replace Sue.
In her first FF mission, Crystal proved her worth, defeating the Wingless Wizard all by herself. For two years, she fought nearly all of the Fantastic Four's rogue gallery including Doctor Doom, the Mole Man, the Frightful Four, the Skrulls, the Mad Thinker, and her own deranged uncle, Maximus the Mad.
When Crystal became ill, Reed Richards realised she and the rest of the Inhumans had never evolved to adapt to the pollutants in human society's atmosphere since they lived in isolation for millions of years. As a result, she was forced to return to Attilan, ending her membership in the Fantastic Four.