The nature of the Titans that walk the forsaken lands beyond Wall Maria are one of the biggest questions of Attack on Titan. While all Titans are gargantuan human-like creatures, there exist massive variations in their anatomy and unique abilities. Pure Titans are mindless, deformed giants whose deadliness varies depending on traits, such as their height and nocturnal adaptations, while Titan Shifters are those controlled by humans and designed to have unique combat specializations of their own. The Titan Eren Yeager controls, naturally, falls into the latter category.Even among the ranks of humans with Titan powers, Eren’s situation is nothing close to simple. As someone who was never trained to wield a Titan his Titan anatomy has both great limitations and great potential.
How Many Titans Has Eren Yeager Inherited?
This first detail neatly sums up why Eren’s Titan is so different from the rest. Eren technically has access to two Titans instead of just one at the beginning of Attack on Titan. Thanks to the efforts of his father, Grisha Yeager, Eren has full access to the powers of the Attack Titan and limited access to the Founding Titan. This was brought about by his father's Attack Titan eating Frieda Reiss, the Founding Titan, and then Eren as a pure Titan ate Grisha.
Titan powers are inherited by eating someone who already has access to those powers, and because of the Eren-Grisha-Frieda chain of consumption, Eren’s Titan abilities pulls from both shifters. This further evolves later on, as he eats the War Hammer Titan in Season 4. Although he physically looks the same in his Titan form, his abilities will inherently change to include elements of the War Hammer Titan.
What Is the Attack Titan's Ability?
Nine Titans can be controlled by humans in Attack on Titan’s world, and throughout the series each one is characterized by the combat specializations unique to that Titan. For example, the Armored Titan has plated segments protecting its body that render it invulnerable to most attacks by humans, and the Colossus Titan stands at a 60 meter height that dwarfs the other human-controlled Titans. However, Eren’s Attack Titan is a strange exception to the rule. The Attack Titan has all the characteristics that are universal to Titans -- with the most important being super regeneration -- but it lacks any unique abilities of its own. When every other human-controlled Titan has its own weird abilities, the lack of a weird ability suddenly becomes even stranger.
How Did Eren Yeager Get His Hardening Ability?
The traits of Eren’s Titan are weird, in large part because they seem contradictory at first glance. Although the Attack Titan lacks any unique abilities at its base, this changed in Season 3 due to a special serum possessed by the Reiss family. The serum upgraded the Attack Titan, allowing Eren to harden his skin into a crystalline substance that is invaluable for both defense and offense.
It’s still not an ability unique to Attack Titan, since in Season 1 the Female Titan naturally possessed it, but it’s arguably the most vital Titan power Eren wields. All Titans carcasses are known to disintegrate once they’re slain or the human controllers are cut free from the body, but in hardened form the crystalline carcass can remain. It’s this anatomical trait that allows Survey Corps to patch up the walls using hardened skin from the Attack Titan.
Why Does the Founding Titan Need Royal Blood?
Eren may have inherited the Founding Titan, but even though he theoretically has access to this Titan’s abilities, he can’t fully use them. The Founding Titan was meant to be passed down through the Reiss family’s royal bloodline. Because Eren lacks royal blood, under normal circumstances he cannot access the Founding Titan’s incredible abilities -- namely memory manipulation and command over hordes of pure Titans. Yet these abilities are not utterly lost to Eren. If he comes into physical contact with someone of royal blood, he’s able to temporarily command Titan hordes and delve into the memories stored within the Founding Titan.
Why Is Eren Yeager Not Always in Control of His Titan?
The final trait on this list is possibly one shared by every human-controlled Titan, but so far we’ve only seen Eren contend with it in the series. If Eren lacks a strong force of will, it’s possible for him to fall into a trance-like state and lose control of the Attack Titan. He nearly killed his adopted sister Mikasa in a state of feral rage when this occurred, and later, he nearly ate Annie, despite the Survey Corps wanting to keep her alive for information. There is a chance that this could be due to Eren's lack of training, which the Shifters from Marley underwent before becoming Warriors.