In fact, a private database of Marvel’s characters has been created just for the show’s writers. There will be some characters incorporated from the Marvel movies, but there are also characters that will be considered off limits. In some cases, the characters are considered off limits just because Marvel wants to reserve their star power for the big screen, but in other cases there are rights issues that will prevent use.
One might suspect that the show would love to use some of the lesser-known mutants from the X-Men universe, but FOX actually owns the live action rights to the X-Men and the Fantastic Four. In fact, not only will the X-Men not appear on Agents Of S.H.I.E.L.D., but their very existence will not even be referenced. "We can never use the word 'mutant,’” said Executive producer Maurissa Tancharoen .
Instead, Marvel has trademarked the words “registered gifted” to use to refer to characters who have special abilities. In the series, Agent Coulson and his team will track down those that they refer to as the “unregistered gifted.” While not confirmed, it’s likely that this same gifted terminology will carry over to the Avengers sequel in describing Scarlet Witch and Quicksilver, because of the same rights issue involving FOX.