|Every country out there is frothing at the mouth, just waiting to get Marley when it's weak. I know, because my family came here from an internment zone in another country. It was awful...The hostility that Eldians face here is nothing compared to the way it is in other countries.|
|— Udo explains global hostility against his people|
Udo (ウド Udo?) was an Eldian Warrior candidate (戦士候補生 Senshi Kōho-sei?, also translated as "Warrior Cadet") working for the Marley government. Before his death, he was a candidate to inherit the Armored Titan power.
Udo was a boy of average height and build, though he was easily the tallest and heaviest of the Warrior candidates, with shaggy black hair and dark eyes which he wore oval-shaped goggles over in place of glasses. Like his fellow Warrior candidates, he wore the standard Marleyan soldier uniform consisting of a light jacket and pants, supply packs on the hip, suspender straps, a hard hat with a stripe around it, and tall combat boots. As an Eldian, his uniform was different from that of the Marleyans, as his shirt collar has a small badge with the Eldian star instead of a small clip and his hat has a white stripe with the Eldian star on it instead of a pure black stripe. As is law in Marley, Udo also wore the Eldian armband as all his family and ancestors residing there do as well.
Udo was not the most secure soldier nor did he possess a very level head. Like his other Warrior candidates, he was trained to aid the wounded and cared enough to come to Falco Grice's aid, but his shaky nature sometimes causes him to neglect those he is treating. While he was not an excelling soldier, he possessed some margin of skill to have survived for four years in the Marley Mid-East War, he also seemed to a be knoweldgeabe about at least one foreign language as he was able to comprehend and interpret the sayings of the wounded Mid-East Allied Forces's soldier.
On multiple occasions, he was shown to be frustrated about the harsh treatment that Eldians are forced to bear, either by his Marleyan superiors or by his enemies on the battlefield.
Udo and his fellow Warrior candidates participate in the Marley Mid-East War. When an injured Falco Grice is brought back to the trench by his brother Colt Grice, Udo and the other Warrior candidates rush to his aid and begin treating his injuries and bandaging his wounds. During this time, Udo begins to panic in fear of their military failing and wraps Falco's bandages too tightly causing, Gabi Braun to scold him.
When Falco returns to the trench with a wounded Mid-East soldier, Udo helps tend to the soldier's wounds. The soldier mutters something in a different language and Udo helps translate what he said.
After the battle, Udo is seen with the rest of the Warrior candidates in Fort Slava's harbor, watching as surviving Mid-East soldiers are sent back to their home countries. Udo grows angry, believing that the soldiers' war stories will only serve to demonize Eldians to the rest of the world, and begins lashing out at inanimate objects around him until Reiner Braun arrives. He is later seen at a party being held in Gabi's honor during the journey back to Marley.
Upon arriving at Liberio, Udo is reunited with his family. When the Warriors and Warrior candidates are summoned to their headquarters soon after returning, Udo complains at how little time they had to rest after the Fort Slava battle.
During training, Udo congratulates Falco for beating Gabi for the first time. On the way back to the internment zone, he enthusiastically shares the news with the soldiers and comments on how exceptional it is. Later that day, he watches the preparations for the festival with Zofia and Gabi.
At night, the candidates serve the ambassadors who came to Liberio. Hearing the guests insult the Eldians, Udo gets distracted and spills some wine on an ambassador from Hizuru. Terrified of being punished, Udo apologizes to her but she calms him down and covers up his mistake. As he asks her about her motivations, he is shocked to learn she helped him despite knowing he is an Eldian.
The following day, Udo enjoys the festival with his friends, and then joins the Warriors to listen to Willy Tybur's speech. When Willy reveals the truth about the Great Titan War during the play, Udo and his fellow Warrior candidates are stunned to learn that the 145th King, Karl Fritz was the one who put an end to the war.
When Eren Yeager suddenly transforms into the Attack Titan behind the stage, debris is hurled into the audience. A flying piece of debris lands on and crushes Zofia, killing her. Shocked, Udo attempts to free her from the rock, but before he can do anything, he is trampled by the panicked audience members trying to escape Eren's Titan.
As the crowd flees, his head is split against the rubble and his neck is snapped by the trampling. Colt and Gabi recover his body, though Colt is unable to say whether or not Udo is dead. They rush Udo to the hospital amidst the chaos. At the hospital, the doctor refuses to give Udo a bed, saying that there is no space and he is already dead. After grieving, Colt lays his corpse down on a sheet outside.
- Falco Grice - Falco and Udo are comrades as Warrior candidates for the Armored Titan power. As such, they have grown closer in the four years they spent at war with the Mid-East Allied Forces and care for one another. When Falco returned from battle wounded and delirious, Udo treated his wounds to the best of his ability, despite wrapping his bandage too tight in the midst of a personal panic.
- Gabi Braun - Gabi and Udo work closely together on the battlefield and show a liking towards one another. Gabi is kind to him as she is to all the other recruits, but they appeared to get in a small spat when Gabi yelled at him for panicking and wrapping Falco's bandage too tightly. However, this does show that the two are close enough to exchange opinions without being angry.
- Zofia - As candidates, they worked well together in the midst of war, but showed very little interaction with one another. Zofia seemed to find humor in Udo's blatant panic and saw him in a friendly yet foolish light, but the two were not unkind to one another. During the chaos from Eren Yeager's attack on the Liberio festival, Udo futilely attempted to free Zofia's already-dead body from underneath a boulder, and he was ultimately trampled to death by the fleeing crowd while attempting to save her.
- Udo ranked thirty-sixth in the Third Character Popularity Poll.
- Udo (独活) is the Japanese name for the plant species Aralia cordata.
- Sketches of Udo included in the Attack on Titan: Final Manga Exhibition revealed that Udo was planned to have the full name Udo Bock (ウド・ボック Udo Bokku?).
- "Bock" is a style of beer that originated in Germany.