Eren Kruger was an Eldian spy posing as a Marleyan officer in the Public Security Authorities. He founded the Eldian Restorationists, and used the pseudonym the Owl to communicate. Secretly, he possessed the power of the Attack Titan.
Kruger was a fairly large, well-built, older man. He had shaggy dark hair which he kept combed to the side. Kruger had a very slender face and pronounced cheekbones, and he was seen wearing the standard uniform for Marleyan soldiers and officers, although he only wore his uniform's hat when he was facing civilians.
Kruger appeared to be a largely stoic individual who never displayed any emotions, most likely as a consequence of enduring years of suffering and misery as a mole and being forced to commit brutal and atrocious acts against fellow Eldians. After finding Grisha and Faye Yeager outside of their internment zone, he felt no qualms in punishing Grisha physically. He again continued to show a lack of empathy when the Yeagers learned of Faye's death, remaining stone-faced at the sight of their sorrow.
Appearance: Kruger was a fairly large, well-built, older man. He had shaggy dark hair which he kept combed to the side.
Personality: Kruger appeared to be a largely stoic individual who never displayed any emotions, most likely as a consequence of enduring years of suffering and misery as a mole and being forced to commit brutal and atrocious acts against fellow Eldians. After finding Grisha and Faye Yeager outside of their internment zone, he felt no qualms in punishing Grisha physically.
History: Kruger was born to an Eldian family living in Marley. His father had joined an Eldian revolutionary army led by the remnants of the royal Fritz family who remained on the mainland.
How Eren Kruger echoes later: Kruger is one of the many Titan inheritors whose Titan is created by Ymir Fritz to defend Eren Yeager while he is attempting to Rumble the world. In the middle of the fighting, Kruger and a handful of other inheritors are reawakened in the Paths by Zeke Yeager. Zeke and Armin Arlert ask the inheritors to lend them their strength.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.