Who was Levi's host? Spoiler alert! Do no proceed unless you are up to date with the manga or you don't mind being spoiled.

In Chapter 112 of Shingeki no Kyojin, according to Eren, the Ackerman are able to unleash their power when their host are in danger and they will often have headaches as their inner self fights against the instinct of the Ackerman blood.

He then told Mikasa that her tagging along wherever Eren goes is just from her Ackerman blood as Eren is her host.

Levi is also an Ackerman and as seen multiple times, he is capable of nearly inhumane feats such as slaughtering a horde of Titans all by himself. My questions is, who was Levi's host? Was it Erwin? If so, do we have proofs that it was him?
For Levi, that person is Erwin. He has acknowledged Erwin as an existence who he looks up to. It’s part of the Ackerman bloodline, or an instinctive part of them, you could say.
  source: Hajime Isayama, book of answers