Agent 33 (real name Kara Lynn Palamas) was a former agent of S.H.I.E.L.D..
When Hydra "came out of the shadows," Agent 33 tried to save herself by hiding out in a S.H.I.E.L.D. safe-house, but Hydra eventually discovered her location and captured her.
Agent 33 later discovers Bobbi gave Hydra the location of her safe-house; at the time, Bobbi was on an undercover mission inside Hydra and in order to earn Hydra's trust and get deeper in the organization she gave Hydra the location of Kara's safe-house, although not knowing if the safehouse was occupied.
Bobbi later admitted that even if she knew Kara was there she'd still give them the safe-house's location as if she didn't then she'd be putting the lives of dozens of S.H.I.E.L.D. agents at risk.
Happy to Comply Edit
In "Making Friends and Influencing People", the episode opens with Bakshi and Dr. Whitehall brainwashing Agent 33 with the Faustus Device. After Bakshi leaves, Whitehall continues to torture Agent 33 for days until, by the end of the episode, she's seen working as an Agent of Hydra being completely loyal to Whitehall.
In "Face My Enemy", Dr. Whitehall sends Agent 33 and Bakshi on a mission to steal a painting with symbols carved into the back which match the symbols on the Obelisk. However, after they learn Coulson and May are also after the painting, Whitehall orders them to capture Coulson.
After they capture May, 33 uses the photostatic veil to impersonate her and then goes to the Bus to get Coulson. 33 lures Coulson into a trap and, as they're leaving the Bus, she leaves behind a device which hacks into the plane's computers and sets it to self-destruct.
Before 33 can capture Coulson, he realizes she isn't May and a fight ensues. After May escapes, Coulson chases after Bakshi and May starts fighting 33. May is able to knock out 33 by electrocuting her face with a live wire.
In "Ye Who Enter Here," it's revealed that the electric shock from May has left Agent 33 with a scar on her face. Also, the electricity caused the photostatic veil to malfunction so now Agent 33 is stuck permanently looking and sounding like May.
Dr. Whitehall sends 33 on a mission to British Columbia to capture Raina, but S.H.I.E.L.D. interferes and take Raina away before they can capture her. At the end of the episode, 33 meets with Whitehall to inform him Ward succeeded in catching Raina, but he's also taken Skye as a prisoner which wasn't part of his mission. Furthermore, Ward didn't shoot down the Bus like he was ordered to; Whitehall then orders 33 to fix the problem and she orders the Hydra jets to shoot down the Bus.
In "What They Become," Coulson kills Dr. Whitehall and with him dead Agent 33 begins to panic. 33 finds Ward bleeding to death on the floor after Skye shoots him; 33 confides in Ward that she doesn't know what to do without Whitehall and Ward tells her that if she helps him then they can figure out what to do together. 33 then carries Ward out of the building and tends to his wounds thus saving his life.
Love & Revenge Edit
In "Love in the Time of Hydra," at the beginning of the episode, Ward and Agent 33 abduct an unnamed doctor from a diner and take him back to a hotel room where he repairs the photostatic veil. The veil still can't be removed from 33's face, but it is functional again allowing her to change her appearance and her original voice is also back to normal. Ward, later, hints that he killed the doctor after the veil was repaired as "it was important he not talk."
Later, Ward tells Agent 33 what his family and Garret put him through and how she needs closure to move on. Ward then hands her a file on Bakshi and explains to her how she needs to make him feel what she feels. Ward and 33 then break Bakshi out of prison and abduct him. Back at their hotel room, 33 uses the Faustus Device on Bakshi to brainwash him into being completely loyal to them.
In "S.O.S. Part One," Ward and Kara abduct Bobbi and when Bobbi wakes up she finds herself in a torture room. Ward then explains to Bobbi how they're going to torture her until she admits what she did to Kara. Bobbi tries to explain to Kara that she didn't know she was in that safe-house and killing her won't bring her closure as Ward we'll just find another person to blame, but Kara doesn't listen to her.
Later, Bobbi tries to escape and fights Ward, but Ward stops her by breaking her leg. Kara is then going to kill Bobbi, but stops when she realizes Bobbi doesn't care if she dies and still doesn't regret letting Hydra take her. Ward then has a new idea; first they turn Bobbi's phone back on so S.H.I.E.L.D. can track it, then they tie Bobbi to a chair and put a gag in her mouth, and finally they attach a motion sensor to the door so a gun will a go off and kill the first person that walks through the door, who Ward believes will be Hunter.
Forcing Bobbi to watch Hunter die and letting her live knowing it was all her fault will give Kara her closure.
In "S.O.S. Part Two," Kara and Ward watch May, Hunter, and several other S.H.I.E.L.D. agents storm the building via the buildings security cameras. Ward then tells Kara they'll kill all the others one by one to make sure Hunter is the first to find Bobbi.
Kara uses the photostatic veil to impersonate May, but when Ward sees her he thinks she's the real May and kills her. Kara dies in Ward's arms.
- Photostatic Veil: After May electrocuted her face while she was wearing the veil, the veil became permanently attached to Agent 33's face. It allowed Agent 33 to take on the appearance of anyone she saw and she could also copy that persons voice too if she had a voice sample of that person.
- In Marvel Comics, Agent 33 was a Greek mythology expert tasked with recruiting Hercules to S.H.I.E.L.D.
- Ward blamed Coulson for Kara's death and, in "Closure," Ward kills Rosalind Price, the woman Coulson loved, as revenge.