Leonhart is an Eldian who lived in the Liberio Internment Zone and is the adoptive father of Annie Leonhart. He had trained his daughter to be a Warrior candidate which later led to her selection as inheritor of the Female Titan's power.
Leonhart has a receding hairline with his dark hair combed back. He appears to be a relatively short man, and well built for fighting as he is a trainer. He has only been seen wearing a light jacket over a white shirt with dark pants and shoes. During the Liberio festival, he is seen wearing a beret and using a cane. He is disabled and unable to walk straight.
He was selfish at first with Annie and his goal was to solely train her into becoming a Warrior of Marley so that he could have a wealthy life. He seemed to be a relentlessly strict trainer, not allowing his daughter to take breaks and yelling at her during the training sessions. However, sometime before Annie's departure, he had a change of heart and apologized to her, he finally saw her as his true daughter saying that she come back someday, and assuring her that he would always be on her side. He seems to be very motivated to see Annie again.
Appearance: Leonhart has a receding hairline with his dark hair combed back. He appears to be a relatively short man, and well built for fighting as he is a trainer.
Personality: He was selfish at first with Annie and his goal was to solely train her into becoming a Warrior of Marley so that he could have a wealthy life. He seemed to be a relentlessly strict trainer, not allowing his daughter to take breaks and yelling at her during the training sessions.
History: Many years ago, Leonhart was relocated to the Liberio internment zone from another country after it was discovered he had Eldian blood. While living in the city, he came across a baby girl who was abandoned for similar reasons; Leonhart decided to adopt the girl and gave her the name Annie.
Leonhart in the story: After impressing Eren Yeager with her fighting technique during training, Annie mentions that her father was the one who taught it to her. She goes on to deride the ideals he held, comparing them to Eren's, and claiming that they have nothing to do with reality. While Annie is trying to escape from Stohess District transformed in her Titan form, she remembers the last words her father said to her before she left Liberio.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.