Attack on Titan boldly shifts its focus away from Titans during Season 3 as the Survey Corps halt their scouting missions in order to overthrow the government. This new direction required fresh faces to explore Levi's backstory, introduce a formidable human antagonist, expose the corruption of the government, and shed new light on the history of humanity, Titans, and Ackermans. It placed a lot of narrative responsibility on the shoulders of one character: Kenny Ackerman.
Formerly known as Kenny the Ripper, he was a prolific serial killer who operated in the capital inside Wall Sina. There is an urban legend, which Levi confirms to be true, that Kenny has slit the throats of over 100 officers of the Military Police. Kenny's base of operations appears to be the Underground, a subterranean, low-income city with a high crime rate.
One of Kenny's more notable character traits is that he raised and trained Levi Ackerman when Levi was a child living alone in the Underground. This extremely personal history between the two is vital because it allows for a human antagonist who could conceivably pose a threat to humanity's greatest soldier. Levi goes so far as to warn his comrades that Kenny is even more dangerous than he is. In a conflict devoid of Titans, Attack on Titan manages to maintain its life and death stakes as Levi barely manages to escape Kenny's ambush.
The Survey Corps' coup d'état is the central focus of Season 3's first half. To prevent their rebellion from being interpreted as seeking power for themselves, it was important for Attack on Titan to demonstrate that these actions were also in humanity's best interest. Again, Kenny Ackerman is the perfect solution. A former serial killer being employed by the Military Police to perform government-sanctioned executions of political enemies is ample proof that the government's corruption runs deep.
The series wisely uses this pause from actual Titans to expand upon the mythology of Titan powers. After failing to kill him, Kenny becomes close personal friends with Uri Reiss, the previous inheritor of the Founding Titan. His relationship with Uri sheds some light on how the Founding Titan works, including the ceremony of passing Titan powers from one generation to the next. Kenny's knowledge and possession of a mysterious syringe that turns people into Titans, which he passes on to Levi just before dying, would play a critical part in the second half of Season 3.
One of the most surprising reveals of this storyline is the revelation regarding the Ackerman bloodline. Kenny's father reveals to him that the Ackermans were once a family of warriors who protected the King. Ackermans are able to manifest the strength of Titans in human form, explaining why Kenny, Levi, and Mikasa are much more capable warriors than the average human. However, the bloodline was nearly wiped out because the King feared their immunity to the mind-altering powers of the Founding Titan.
While confirming that Kenny, Levi, and Mikasa are all descendants of the same bloodline, Kenny also reveals why he decided to raise Levi in the first place. As it turns out, Levi's mother was Kenny's sister, who died when Levi was very young. As his only living family member, Kenny felt he owed it to his sister to look after her son, his nephew. Once Levi was able to fend for himself, Kenny felt he had fulfilled his obligation to his sister and left him.
Kenny Ackerman had a remarkable narrative responsibility, doing a lot of the story's heavy lifting during his stint on Attack on Titan. It's an impressive feat to have a character be central to so much exposition but also have his own identity. He appears to have held at least some affection for his father and sister, but not so much for Levi, considering he tried to kill him several times during Season 3. He had a best friend and goals, making him about as well-rounded a character as he could have been -- and he wasn't even a Titan.