The Tybur family is a house of Eldian nobles living in Marley, however, in truth they are the nation's highest authority. Even though they are Eldian, they are highly respected throughout the world for being the first Eldian family to fight against Karl Fritz and fight to restore Marley in the Great Titan War. The family gave Marley freedom and power as an act of atonement for their ancestors' crimes. Willy Tybur was the head of the family.
They kept the War Hammer Titan for generations, but only a select few knew of the previous inheritor's identity.
History: Before the Great Titan War, the Tybur family was one of several Eldian houses warring for control over the Nine Titans. By the time war had broken out between Eldia and the remnants of Marley, the Tybur family had possession of the War Hammer Titan. Opposed to the 145th King of the Fritz family, the Tybur family was the first of the Eldian houses to side with Marley in the war.
Story: In the aftermath of the Marley Mid-East War, Zeke Jaeger grows concerned that the Eldian race has been put at risk with the world's growing technological advantage over the Titans. In order to gain the support of the viewing world for the next mission against Paradis Island, the Tybur family agrees to cooperate with the Warriors by holding a press conference in Liberio during an upcoming festival, where they plan to announce that the island of Paradis will be overtaken within one year. Later on, the family visit Liberio.
Trivia: Tybur appears to be a variant spelling of the name "Tiber", which is itself a variant of the Latin name "Tiberius". "Tiber" is also the English name of the river which runs through Rome and the third largest in Italy.
Source: Tybur family (Anime), available under CC BY-SA 3.0.