At the brink of death, Kenny confesses that he is Levi's uncle, explaining that he left Levi because Kenny viewed himself not fit to be a father. As a final gift, Kenny gives Levi the Titan injection that he stole from Rod. Soon after, he dies, sitting under the tree where he rested.
When Levi was first introduced, he was one of the most mysterious characters in the series, with people not even knowing what his last name was. However, after Kenny revealed to him that he was his uncle in his final moments, Levi found out that he was an Ackerman.
So according to the man himself, Levi did feel sadness and a sense of loss at the death of Kenny. Learning Kenny's identity, that he was his uncle and not his father, gave Levi relief.
Since he uses the term "branch" it's likely not his immediate family (parents and siblings). I figure they're probably cousins, perhaps distant ones, since he doesn't bother with names. Kenny also mentions his sister's name, Kuchel, so definitely not Mikasa.
There has been much speculation over the relationship between Levi and Mikasa Ackerman, with some people believing that they are related. However, both have denied any such connection and insist that they are just friends.
A few episodes before that he is actually surprised Kenny's last name is Ackermann. But after doing some digging i found out that in the Manga during the cart scene Levi also mentions he is an Ackermann.
Levi Ackerman (リヴァイ・アッカーマン Rivai Akkāman?), often formally referred to as Captain Levi (リヴァイ兵長 Rivai Heichō?), is the squad captain (兵士長 Heishichō?, lit. "leader of the soldiers") of the Special Operations Squad within the Survey Corps and is widely known as humanity's strongest soldier.
Kuchel Ackermann (クシェル・アッカーマン Kusheru Akkāman?) was the younger sister of Kenny Ackermann and the mother of Levi Ackermann. She worked as a prostitute in the Underground.
Kuchel Ackerman (クシェル・アッカーマン Kusheru Akkāman?) was the mother of Levi Ackerman and younger sister to Kenny Ackerman.
It is later revealed that Kenny was Levi's uncle, who took care of the latter after his mother, Kuchel Ackerman died. After teaching Levi how to fight, he abandoned him feeling that he could not be a good parent to Levi.
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.
⁵ Introduced in Year 854. ⁶ In the Year 829, Kenny was a young man (and serial killer), so it can be assumed that he was around 20 at this point.
He's Mikasa's uncle by her mother. "Attack on Titan" creator Hajime Isayama doesn't reveal Levi's age but he did say Levi is “surprisingly old.” Also, Mikasa's father was tall and blonde—nothing like Levi.
Smoke Signal (Episode)
He seemed to derive amusement from taunting Levi, making fun of his stature, and degrading his choice to join the Scouts. In his final moments, whether it was out of respect, regret, or because he wanted to attempt at least one act of compassion, Kenny left Levi the case of titan serum he stole from Rod Reiss.
The Ackermans have an "awakened power" displayed by some members of the clan. Levi Ackerman describes it as knowing exactly what needs to be done, and it lets the Ackermans exhibit physical abilities much, much higher than the average human.
Mikasa Ackerman - His only daughter, he did what he could to protect her from the government's persecutions.
Chapter. Ackerman (アッカーマン Akkāman?) was the father of Mikasa Ackerman and the husband of Mikasa's mother.
Who was the titan shifter that was with Kenny? Uri Reiss (ウーリ・レイス Ūri Reisu?) was the younger brother of Rod Reiss and the uncle of Historia Reiss. He was the true king of the Walls for some time until the year 842 and was Kenny Ackerman's best friend.
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.
Traute Caven (トラウテ・カーフェン Toraute Kāfen?) was the second in command of the Anti-Personnel Control Squad within the Military Police Brigade and was very loyal to her superior, Captain Kenny Ackerman.
Who is Kenny to Levi and Mikasa? Levi is the son of Kuchel Ackerman, sister of Kenny Ackerman. Their grandfather had two or more sons, one of which, of course, was Kuchel and Kenny's father.
Mikasa Ackerman credits: Levi's wife (Real) - YouTube.
In some apocryphal texts such as the Testaments of the Twelve Patriarchs, and the Book of Jubilees, Levi's wife, his children's mother, is named as Milkah, a daughter of Aram.
While there are many characters that he respects, Erwin Smith is perhaps the only character that Captain Levi has truly loved, which puts Erwin at the very top of the list. Levi's loyalty and devotion to Erwin also indicate that the two were meant to be together.