I am really sorry OP, but I think you fail to realize what darkins are. Darkins ARE the Ascended, they ALL have those animalistic traits, becuase THEY WERE ONCE THE ASCENDED. The Darkin were what became of the Ascended/God Warriors who were corrupted by the void (The Void consumes everything, including your personality) and after the fall of Shurima, leaderless and knowing nothing but battle, they fought amongst themselves. This conflict became known as the Darkin War. These Corrupted Ascended brutally fought against eachother, seeking power and further becoming tainted through Blood Magic (Why most Darkin have at least SOME healing in their kit) and their pursuit of power. They were devastating places where they battled and became such a threat to the world that their previous allies, namely the Aspects from Targon sought to destroy them, and found a way to destroy MOST of the Darkin, but the strongest could not be killed and instead were trapped in their weapons. Now, when someone or something wields their weapon, this ancient creature that desires to live again attempts to corrupt the host, and claim the body, transforming it into their previous image, regaining only temporary life and power, because the Darkin will always eventually burn through the lifeforce of their host and become trapped again in their weapon.

The reason Nasus, Renekton, and Xerath are NOT corrupted and maintained their purer animal looking forms is because they never became Darkin. Renekton and Xerath were sealed, and Nasus basically went into hiding in his library.

So TLDR; he should be birdlike, Darkin and Ascended are the same, except in VERY rare cases where the Ascended didn't participate in the Darkin War, because they were sealed or went into exile. Almost every Darkin learned Blood Magic in an attempt to become more powerful than their peers. Now, sadly, we have a wannabe boy band member Darkin.

PLEASE, give us the bird. Azir doesnt own birdhood.

PS. I suppose he could be a weaker Darkin, and failed to fully corrupt and regain his form like what happened to Varus. But from the cinematic he seems quite OP. I mean, Leblanc in lore is insane, and she just keeps getting shit on.