Most characters on Agents of SHIELD went through pretty serious trauma. Daisy (Chloe Bennet) was followed by death and destruction since she was a baby and openly struggled with that. Melinda (Ming-Na Wen) was forever changed by one horrible mission. Fitz (Iain de Caestecker) experienced brain injury, hallucinations and even death.
Then there was Mack (Henry Simmons), who lost his daughter and Elena (Natalia Cordova-Buckley), who lost her arms. And that's without mentioning Jemma (Elizabeth Henstridge), who was kidnapped at least once per season and even spent a stint as a technologically-deafened space slave. The main characters got to experience highs along with their lows, however -- which wasn't the case for Kara (Maya Stojan).
Kara Lynn Palamas, aka Agent 33, first appears in season 2, episode 3. A SHIELD agent, she's first shown being tortured by Daniel Whitehall (Reed Diamond). How she was captured isn't revealed until much later in the season, but it stands to reason that she was a victim of the Hydra takeover. Her first scene is also the only time we see her express what is unarguably her own mind, and she says very clearly, "I will never work for Hydra."
Unfortunately, the next time she appears, later in the episode, she's been successfully reprogrammed by Hydra and is serving as Whitehall's new right hand. In the next episode, she dons a nanomask that makes her look like May, then gets disfigured by May herself in an epic fight. Whitehall still keeps her around as a bodyguard of sorts, but the nanomask has been fused onto her face, meaning she wears May's face for several episodes.
When Whitehall dies, she's ostensibly rescued -- but by Ward (Brett Dalton), arguably the main villain of the show's first three seasons. He treats her kindly, vowing to help her find her real identity and initially refusing her romantic advances because he doesn't want to take advantage of her. The scene where he does so is particularly dark because Kara, using her nanomask, has shifted her face to look like Daisy's, knowing that Ward has feelings for the other woman. However, it all looks to be going well at first, with Ward helping her get revenge against Bakshi (Simon Kassanides) and get photographic evidence of her real face.
The revenge is satisfying and Kara getting to reconnect with her identity is good, but Ward's treatment of her is deeply insidious: he's acting like he's helping her get her identity back, but (partially through an eventual romantic connection) he's actually gaslighting her. She takes on his anti-SHIELD viewpoints and unquestionably believes the narrative that he's just a victim. When she eventually gets found by SHIELD, Jemma is able to remove the actual Hydra brainwashing from her, but the more psychological conditioning from Ward remains.
This culminates in Ward and Kara kidnapping Bobbi (Adrianne Palicki) and torturing her because she, while undercover in Hydra, told Whitehall about the location Kara was found in. She didn't know Kara was there and might not have even known Kara had been taken until the brainwashing had already been completed, but Ward, being anti-SHIELD, sees this as an opportunity to give Kara "vengeance" while destroying SHIELD agents. They do horrific things to Bobbi while Kara blindly espouses the rhetoric that Ward fed her.
When other agents come into the warehouse where they're holding Bobbi, Kara shifts her nanomask into May's face, and Ward shoots and kills her by accident. At that point she hasn't actually gotten justice or vengeance; she's barely even been shown actually acting as herself. There are hints about her life and relationships before her kidnapping -- she and Bobbi actually have a conversation when she's mid-deprogramming that suggests that they were sort of friends -- but very little is explicitly revealed, and she never gets to take her life back.
Her role in the story is strikingly similar to Bucky's (Sebastian Stan) role in the greater MCU -- former hero brainwashed into working for Hydra -- but while he's loved and lauded, she's mostly forgotten. This could be because she was a minor character in one season of a show, while Bucky appears in the more-popular movies, or it could also be because she's an Asian woman and not a white man.
Other characters, particularly Bobbi and Jemma, are shown to have sympathy for Kara in canon, but after she dies, she is mostly forgotten. When she's brought up later, it's in the context of being Ward's dead girlfriend (though May does, rightly, mock him for having accidentally killed her). She's not just forgotten but misunderstood, which adds to the pure tragedy of her story.