When Yuri arrives in Shin Makoku, he is declared the 27th Maoh of the Mazoku ("Demon Race"). Yuri easily gets caught up in his emotions to the point of losing control of himself. He is a little bit quick to anger, and once he's on a rampage he can't even stop himself. A music teacher he once had dubbed his rants "the Turkish march". When he is angry or sad or feels a strong emotion his powers emerge. Yuri uses expressions from historical dramas when he gets this way (since he is a fan of historical dramas) They call this condition is referred to as Maou mode (上様モード (uesama mo-do/emperor mode) in Japanese). In the anime this form can be recognized mainly by the longer hair and demon eyes. He can summon powerful magic (majutsu), even in the human territories (a notable feat as most Mazoku cannot perform magic outside of the country), and even more remarkable is that he can use magic (majutsu) in souseki filled areas such as the souseki mines where other mazoku have difficulty standing and even breathing. Yuri's magic is manifested in various shapes - most notably as one or more great water dragons, but he has made messy magical "creatures" made from mud, food morsels etc. After using his magic, Yuri becomes worn out and in some cases passes out and doesn't remember what happened afterwards. Later, Yuri learns to control his power and uses it to close the first of the Forbidden Boxes. In the beginning of season 1, he often fainted after he used magic (Maryokou) due to exhaustion. But later on he was able to maintain his consciousness after returning to normal.
Yuri is in fact the reincarnation of Susanna Julia von Winncott who died during the war against humans 20 years ago. The pendant Conrad gave to him belonged to her. Since Adalbert brought out his stored language memory in his soul, he can understand the mazoku spoken language but because Suzanna Julia was blind he is unable to read the language. Although, he can read raised letters with a finger. There are times in the novels where when he goes into a panic mode Julia's influence takes over a bit and he temporarily loses his eyesight.
Yuri has many allies and friends. His closest allies are Wolfram, Conrad, Günter, Gwendal, Murata, and Yosak. They help Yuri throughout the series. During his first encounter with Wolfram von Bielefeld he ends up engaged to him due to a mishap: Wolfram, angry at the idea that Yuri is supposedly the Maoh, makes the mistake of insulting Yuri's mother - which earns him a slap. Slapping another noble on his or her left cheek is the way in which Mazoku propose marriage (some consider the slap as unmanly, but Yuri explains in the light novel that he only slapped Wolfram because he has a beautiful face, otherwise he would have punched him). Incensed by this insult, Wolfram challenges Yuri to a duel, which Yuri also mistakenly accepted because of his unfamiliarity with Mazoku custom (he picked up the knife that fell off the table).
Throughout the series, his main goal is to create a world in which Mazoku and humans can live together in peace, something that progresses slowly but surely as the series goes on.
In the anime, Yuri is said to be the greatest Maoh after defeating Soushu and freeing the Original King from the dark power. Because of that, Yuri is able to travel between worlds even without the help of the Original King.