The backstory of Attack on Titan's Mikasa Ackerman is secretly even more tragic than many viewers realize. The long-running anime series (as well as the manga it's based on) has never shied away from tragedy or graphic violence, and much of the story's darkness comes not from the giant man-eating Titans, but from man's inhumanity to man. In the cruel world of Attack on Titan, oftentimes humans are the greatest villains, and nowhere is this more brutally realized than in the origin story of Mikasa Ackerman.
Alongside main protagonist Eren Yeager and their childhood friend Armin Arlert, Mikasa is one of the central characters of Attack on Titan's story. The tough-as-nails Survey Corps warrior quickly becomes one of history's greatest Titan slayers, and uses her focus and calm demeanor to accomplish any mission. However, beneath her seemingly cold exterior, Mikasa is a well of chaotic emotion, with her defining trait being her unwavering devotion to the people she cares about, especially Eren.
Mikasa's strength, focus, and devotion are a result of a tragic event in her past, in which a duo of men murdered her parents and kidnapped her when she was a teenager. It was only through the intervention of Eren, and Mikasa fighting back, that she was saved, resulting in her forming a deep bond with the young man she considers to be her savior. However, thanks to a plot point found in the Attack on Titan spinoff story Lost Girls, it's known that Mikasa lost even more of her family on that fateful day than many viewers of the series initially realized.
Initially beginning as a novel before being serialized as a manga in 2015 and then adapted into an anime OVA beginning in 2017, Attack on Titan: Lost Girls focuses on the characters of Mikasa and Annie Leonhart, with three stories detailing elements of their respective pasts and their interactions with one another. The Mikasa Ackerman story, called "Lost in a Cruel World" and largely centered on Mikasa's relationship with Eren, includes some additional information about the day her family was murdered. One of the biggest revelations of the story is the fact that Mikasa's mother was actually pregnant at the time of her death, and would've given birth to a baby boy if she had lived.
This extra detail adds a whole new tragic dimension to Mikasa's story. Not only did she lose her parents, but also a little brother that she never got to meet. With this knowledge in mind, it's easy to see why Mikasa formed such close attachments with Eren and Armin, and it's entirely possible that every relationship she's had throughout Attack on Titan has in some way been informed by this horrific loss.