Special guest star Jaimie Alexander portrays Sif, also reprising her role from the film series.

Through social media they discover footage of Sif fighting a surprisingly strong man whose weapon removed her memories.

In the ensuing fight, Morse is thrown across the room, while Skye loses control of her new and secret earthquake-powers, allowing the Kree to escape.

to the Romanic City of Chaves (English: Keys), in the north of Portugal, where Daniel Whitehall found the Obelisk, or the Diviner (which gave Skye her abilities) during the dictatorial regime sympathetic to the Nazi ideology Estado Novo.

The Kree, named Vin-Tak, cooperates by restoring Sif's memory and explaining his mission on Earth: he had learnt that Terrigenesis, the transformation of a human with special genetic material into a Kree war-slave, had recently taken place on Earth, and had come to put down the "abomination" and destroy the Diviners remaining on the planet.

Skye, unable to stop the shaking, uses a tranquilizer on herself, and when she wakes up, moves into the vibranium-laced cage away from the other agents, who fear her new abilities.

Sif returns Vin-Tak to his homeworld of Hala and promises that the Kree will no longer be an issue for Earth, though she warns Coulson that Skye may be even more dangerous than he thinks.

In February 2015, Marvel announced that the twelfth episode of the season would be titled "Who You Really Are", to be written by Drew Z. Greenberg, with Roxann Dawson directing.

"[2] In February 2015, Marvel revealed that main cast members Clark Gregg, Ming-Na Wen, Chloe Bennet, Iain De Caestecker, Elizabeth Henstridge, Nick Blood, and Adrianne Palicki would star as Phil Coulson, Melinda May, Skye, Leo Fitz, Jemma Simmons, Lance Hunter, and Bobbi Morse, respectively.

[1] It was also revealed that the guest cast for the episode would include Henry Simmons as Alphonso "Mack" Mackenzie, Jaimie Alexander as Sif, Eddie McClintock as Vin-Tak, Alexandra Manea as nurse, Lidia Porto as head nurse, Danny Lopes as Bruno, Ben Griesse as Tiago, Ruben Rabasa as Mr. Cardozo and Markell Andrew as officer Carvalho.

The species plays a significant role in the film Guardians of the Galaxy, where the idea of more peaceful members of the race, like Vin-Tak, was established.

The different artists were once again chosen to create the teaser posters, based on their previous work and how it connected to the themes and emotion of the intended episode.

[8] For "Who You Really Are", the producers enlisted Marcos Martín to create the poster, which highlighted Sif's return to the show, as well as an "inside look" at S.H.I.E.L.D.