Warning: Spoilers ahead! This article will talk about details in the Attack on Titan manga and anime that might give you a shock especially if you’re still in the first parts of the series. We suggest you turn back now if you don’t want to be spoiled.
One thing is for sure: the Ackermans cannot turn into titans. However, they do have inherent strength and capabilities similar to those of titans.
At one point in the manga, Hajime Isayama, the mangaka/author of Attack on Titan revealed that the Ackermans were formed because of an Eldian experiment. The Eldians wanted a protector of the Royal throne, someone with the strength of a titan, but who is not actually a titan. Basically, a titan in human form. These “protectors” were the Ackermans.
The role of the Ackermans was established further when Eren confronted Armin and Mikasa during that one table scene in the restaurant. Eren suggested that Mikasa never actually loved him, that she was just doing it out of duty (duty being: protecting Eren) since it was the purpose of an Ackerman.
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Hajime Isayama never made it explicitly clear why the Ackermans weren’t able to transform into titans, but it’s probably because are a different race, or, a genetically modified Eldian.
The Ackermans do not have access to the Coordinates (the galaxy-like place we see in the Anime), where all Eldians are connected. They also cannot be affected by the Founding Titan’s mind control, so this means that they are biologically (and possibly racially) different from Eldians.
When Mikasa awakened her power as an Ackerman, she had one thing in mind: To fight and protect. The only person who was with her during that time was Eren, and because he was going to get killed, Mikasa gathered all her courage and awakened her powers which saved Eren. Ever since then, all she’s ever done is protect him.
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Again, nothing is ever confirmed by Hajime Isayama, and this question is still one of the unanswered questions of the series. But because the Manga ended last April 2021, we will never know the answer.