#heishichou

It is true that the term Heichô (兵長) is what his comrades call him and address him by, however his official profile on the manga website states that it is simply that. As far to the reason why they simply call him “Heichô" Instead of his official title, has not been explained. I have heard of others opinions saying that it could be Levi not really caring for formal titles or even people using it to simply shorten it.

Levi Ackerman’s official profile from the manga site states:

Levi

兵士長 (heishichou) of the Scouting LegionA person who is viewed as humanity’s strongest soldier,called ‘兵長’  (“Heichô”) by his comrades.160 cm / 65 kg

Heichô is actually a rank from the Imperial Japanese Army and can be translated to 'Lance Corporal.' However that is actually the lowest rank for a non-commissioned officer.But notice, none of the other characters have an actual rank in the original Japanese. Its more of, they all go by what their roles are. Erwin who is Danchô (団長) - Leader of the legion, and Hanji who is titled Buntaichô (分隊長) - Leader of squad.And so we come to heishichô (兵士長) - Leader of the soldiers.Now in fairness Heishichô is not an actual rank but is more a common title given to characters in medieval-themed fantasy works and is usually translated to “Captain" But just as was stated before, the other characters don’t go by official titles but more of what their roles are.