Uri Reiss was the younger brother of Rod Reiss and the uncle of Historia Reiss. He was the true king of the Walls for some time until the year 842 and was Kenny Ackerman's best friend. He inherited the Founding Titan from his father, and passed it along with the crown to his niece, Frieda Reiss.
He was a frail man who usually wore a simple light colored robe. In his youth he had neck length blond hair which would eventually grey with age. He also had grey, weary eyes with a calm, wise-beyond-his-years expression. After inheriting the Founding Titan, Uri's eyes would change between their normal color to a dark, vacant look, which meant he was bound by the will of the First King. Like his elder brother, he was rather short.
Despite his upbringing and his responsibilities, he was a humble man. He refused to hold a grudge against Kenny, despite the latter's rudeness and repeated attempts to attack him; believing that because of the Reiss family, the Ackerman's hate was justified. He even went so far as to bow down to Kenny and formally apologize.
Appearance: He was a frail man who usually wore a simple light colored robe. In his youth he had neck length blond hair which would eventually grey with age.
Personality: Despite his upbringing and his responsibilities, he was a humble man. He refused to hold a grudge against Kenny, despite the latter's rudeness and repeated attempts to attack him; believing that because of the Reiss family, the Ackerman's hate was justified.
History: Uri was the younger brother of Rod Reiss and the second son of his and Rod's father. As their father was the inheritor of the Founding Titan, the "invincible power" that could stop the terror of the Titans once and for all, Uri and Rod pleaded with him to free humanity from their encasement, but he always refused to do so or to explain why he would not.
How Uri Reiss echoes later: Many years later, as Rod is trying to convince Historia to eat Eren and become the next Founding Titan, he tells her about Uri, their unsuccessful attempts to convince their father to stop the Titans, and how Uri had requested him to pray before receiving the Founding Titan. Rod prays to Historia in the same fashion, but she does not believe his story and throws him to the floor, calling him a liar. Later, as Kenny is at death's door, he begins recounting earlier memories in his life; with a majority of them consisting of Uri, from the day they met until his death during the Reiss succession ritual.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.