Here's how Mikasa Ackerman has royal heritage in Attack on Titan - and why Levi doesn't. Since Attack on Titan season 4 began, Eren, Mikasa and Armin's narrow world has become exponentially larger, with the acclaimed anime series expanding into the land of Marley and beyond. After fending off the invaders led by Zeke Yaeger and wiping out the island's roaming titans, the Survey Corps. are now plotting with their former enemy to deliver the entire Eldian race from oppression, one way or another.
Between Attack on Titan seasons 3 and 4, Zeke made contact with the Survey Corps. through his lead follower, Yelena, who sneaked aboard a Marleyan expedition to Paradis and proposed a way the Islanders could fight back against their long-time enemies across the sea. Paradis will use the Founding Titan currently within Eren and the royal blood within Zeke (then after his death, Historia) to threaten Marley with the Rumbling - releasing the millions of titans stacked within Paradis' three walls. This drastic measure is intended to buy the Survey Corps. time to build their forces and gain allies in the outside world, ready to defend themselves without the Rumbling. To this end, Yelena has brought over the Azumabito clan - an ancient family from Hizuru in the outside world. According to the Azumabito family head, the secret mark on Mikasa's arm is a sign that she's the descendant of Hizuru's Shogun family.
As revealed by Kiyomi, the Asian Azumabito clan were allies of the Fritz family long before the walls were built, and even during the Titan War between Eldia and Marley. A son of the Shogun's family decided to follow Fritz to Paradis, and over the next century, continued the Asian bloodline behind the walls. Since the Azumabito clan are immune to the Founding Titan's memory-altering ability, most of his descendants were killed by later rulers after Fritz, and with her mother killed violently by criminals, Mikasa is now the last surviving member of that royal line. This revelation explains the comment about Mikasa's race in Attack on Titan season 1, and also why others of her kind are so rare on Paradis Island.
Of course, Mikasa Ackerman shares a surname with both Levi and Kenny, and the former is even present when the Azumabito clan arrive on Paradis Island. Why is Mikasa considered the rightful successor of the Hizuru shogun, but Levi is out of the picture entirely? This is because Mikasa's Asian Azumabito clan heritage derives entirely from her mother's side. The Ackerman lineage, on the other hand, comes from Mikasa's dad. Plenty of questions remain over the history of the Ackerman clan, but it's known that Mikasa's ancestors on her father's side were experimented on to make them stronger. Her connection to Levi and Kenny comes from this side, meaning those two characters have nothing to do with the Azumabitos. Confusing the matter, the Ackerman and Asian clans on Paradis also became allies, and both served under King Fritz to begin with. Mikasa wasn't the first time these two powerful families crossed paths.
The idea of Mikasa taking charge of a nation in the outside world would feel like a massive departure for her character. Usually reserved until provoked and quietly fierce when protecting her close friends, Mikasa has shown little interest in leadership or nobility, and will surely avoid taking her rightful position of Shogun if at all possible. However, Mikasa has also shown an unerring dedication both to Eren Yaeger, and to the Survey Corps., and if taking charge of the Azumabito clan will benefit her nearest and dearest, that could be where her Attack on Titan story will end.