Attack on Titan

 Attack on Titan' (Shingeki no Kyojin'', literally "The Advancing Giants" in Japanese) is a highly popular manga and anime created by Hajime Isayama. It's primarily known for two things: The titular Titans, which are bulding-sized humanoid monsters that eat people, and a very high body count due to said Titans. The manga was first released in 2011; the anime premiered in 2013, and FUNimation released an English version in May 2014.

Basic Plot and Themes
Attack on Titan takes place in a post-apocalyptic, dystopian Earth where the Titans have overrun the planet for at least a hundred years. The remains of humanity live in an enormous walled city whose walls are built specifically to keep the Titans out. (There are three walls; Wall Maria, Wall Shina/Sina, and Wall Rose.)

The plot kicks off in 845, when a much larger Titan than normal breaks through the perimeter wall at the village of Shiganshina. Its "smaller" kin get through, and bloody chaos ensues. Fortunately, our three heroes--hot-blooded preteen Eren Yeager, his adopted sister Mikasa Ackerman, and his bookish best friend Armin Arlert--escape to an inner district unhurt (but emotionally scarred). They join the Survey Corps, a branch of the military dedicated to venturing outside the walls and fighting the Titans, a few years later, which gives Eren a good place to  both channel his rage issues and get revenge for his dead mother at the  same time. (A Titan ate her.)

Attack on Titan, besides being about badass soldiers who kill giants, is about how people deal with failure (which is fairly common on the show) and change under extreme stress. This means that most of the characters really, really need some kind of psychotherapy.

Key Characters
This show has a lot of characters, so only the ones most relevant to the plot go here.

Eren Yeager
The protagonist. Eren has all the subtlety of a sledgehammer and never, ever gives up. When he joined the military, he vowed to kill every single Titan with his own hands. Thanks to his Titan shifter abilities, he can achieve his goal in a literal sense. Eren is the first shown Titan shifter, and his alternate form is known as the Rogue Titan, as it/he targeted other Titans and not humans.

(A Titan shifter is a human who can transform into a Titan. Their transformations are triggered by self-harm and having a specific goal in mind. All Titan shifters heal quickly, even in human form, and transform into Abnormal Titans.)

Mikasa Ackerman
Eren's adopted sister and Armin's friend by proxy. Mikasa rarely shows her emotions, although she becomes noticeably less stoic if Eren is involved. She's the top graduate of her class and considered the second strongest soldier in the army.

Mikasa's half-Asian ethnicity gives her an exotic beauty (according to her fellow soldiers), since the "orients" are all but extinct.

Armin Arlert
Eren and Mikasa's childhood friend. Less physically capable than either of them, he makes up for it with intelligence and a gift for planning successful maneuvers. (As an example, Armin is the one to discover that the Female Titan, another Abnormal, is controlled by not only a human but by someone in the military; namely, his classmate Annie Leonhardt.) He is the only character with any knowledge of the world outside the walls thanks to a book left by his grandfather.

Armin originally sees himself as a burden to his stronger friends, but gains confidence and a more steely attitude over the course of the series.

Levi (Ackerman)
Captain/Lance Corporal in the Survey Corps and humanity's strongest soldier. He somehow manages to be even more stoic than his sibling Mikasa...except where dirt is concerned. Levi leads an elite squad of Survey Corps soldiers specifically constructed to keep Eren's Titan shifting in line; it's composed of cocky Auruo Bossard, cheerful badass Petra Ral, relatively stable Eld Jinn, and quiet Gunther Schultz.

Hange Zoë
A Survey Corps squad leader/mad scientist whose primary focus is on Titan research. Hange's work is the reason humanity knows how to fight the Titans with any degree of effectiveness. Naturally, he/she (the author never specified) views Titan shifters like Eren as key subjects in the quest to understand Titans.

In Fanfiction
Fangirls tend to think most of the male cast members are hot, which leads to pages upon pages  of yaoi and reader inserts. (Popular yaoi parings are Jean Kirschtein/Marco Bodt, Levi/Eren, Erwin/Levi, Eren/Armin, and Bertholdt Hoover/Reiner Braun.) There are also two prominent yuri pairings: Ymir/Krista Renz and Mikasa/Annie. Modern AUs are also popular, probably so the writers can keep their stories suitable for a wide audience. There is evidence for and against multiple pairings in canon, so agents can only charge for a certain pairing if it's demonstrably out of character for either party.

Attack on Titan is currently going through a fanfic explosion.

In the PPC
There are no agents from the Attack on Titan continuum currently serving in the PPC, nor any missions in this universe. (Yet.)

Minis from this continuum are mini-Titans. Only two have been spotted so far: Eran and Eren Yager, the latter of whom is currently owned by Agent Rayner Blitzkrieg. Mini-Titans, unlike normal minis, eat live pig, live chicken if pig isn't available, and screaming fangirls. However, as Titans do not have a digestive tract...this can get messy.

It's worth pointing out that some characters' names are spelled differently depending on who translated the anime. Agents working in Attack on Titan are advised to be familiar with all official transliterations so they don't charge incorrectly.

They are further advised ''' not to engage in Titan combat, at all, ever. ''' Yes, even if the Sue is a Titan shifter.