|◄ Preceded by|| Tom Ksaver|
|Followed by ►|
|Unknown|| Zeke Yeager|
|I tried to find...my lost son in you. I tried to run from my sins using my Titan power. How much better it would all have been...if I was never born into this world?|
|— Ksaver expresses his regret|
Ksaver had short dark hair and a round face. He wore the traditional apparel of a Warrior and a red armband, showing that he was an Eldian and an honorary Marleyan. Ksaver also wore glasses that Zeke Yeager would later wear as an adult.
Possessing an inquisitive mind, Ksaver was always curious about how the Titans came to be and mentioned to a young Zeke Yeager that he joined the Titan Biology Research Society primarily because of his curiosity. Usually seen with a smile on his face, Ksaver was an easy-going man with a slight care-free attitude; he was also compassionate and comforted Zeke after the boy turned in his parents to the Marleyan authorities.
Underneath Ksaver's friendly nature, he also harbored feelings of self-loathing, along with a desire to kill himself as a means to atone for the part he unknowingly played in the deaths of his family. He also seemed to care little about the ideals of the Warrior Unit by calling it stupid to shorten one's lifespan just to invade other countries.
In his early adulthood, Tom Ksaver lived as a resident of the Liberio Internment Zone. In the year 817, he ran into Grisha Yeager and Faye Yeager as the children watched an airship pass overhead. He and Grisha would at times face violence and prejudice from the Marleyan residents of Liberio for their ethnicity.
At some point in his adulthood, Ksaver fell in love with a Marleyan woman. Knowing that it is forbidden for Eldians and Marleyans to marry, he purposefully hid his Eldian armband whenever he saw her and the two soon got married. Several years after Ksaver's wife gave birth to their son, she managed to discover his Eldian heritage and promptly killed both their son and herself in shame. Ksaver discovered them shortly afterwards and was overcome with grief. Ksaver would later become a Warrior as a form of suicide and eventually inherited one of the Nine Titans, the Beast Titan. He also became a researcher in order to study the Titan powers.
Roughly halfway through his term, Ksaver began observing the new batch of Warrior candidates who would potentially inherit his Titan. One day while riding to the training area with Instructor Theo Magath, Ksaver took particular interest in one who was lagging behind and was told that the boy's name was Zeke Yeager. Although Magath did not respect the boy because of his poor performance, Ksaver did not share his low opinion. Ksaver would eventually meet Zeke in person while playing catch by himself. After a bit of encouragement, he managed to convince Zeke to play with him, complimenting the boy for his pitching ability. Ksaver would continue to bond with Zeke through their games, even explaining how he believed conflict was ridiculous compared to the mystery of the Titans.
One day, Ksaver found Zeke crying in the alley where they usually played catch, and was horrified to learn that the boy's parents were leaders of the Eldian Restorationists movement. With the Marleyan authorities close to discovering the identities of the leaders, Ksaver suggested that Zeke turn in his parents before they were found out in order to prove his loyalty to Marley. Although Zeke was reluctant, Ksaver convinced him by reminding him of all the emotional turmoil the boy had faced as a result of the pressure his parents put on him. When Zeke exposed his parents as members of the Eldian Restorationists, Ksaver was with him and his grandparents as Grisha and Dina Fritz were detained by the Public Security Authorities.
Ksaver continued to play catch with Zeke as the boy progressed through the Warrior program. One day, Ksaver revealed to Zeke that he had discovered through his research that the Founding Titan, in addition to memory alteration, was capable of changing the very body composition of Subjects of Ymir as well. Zeke suggested that if that were true, the Founder could be used to sterilize all Subjects of Ymir and eradicate the Subjects of Ymir in about a hundred years; after a brief pause, Ksaver then told Zeke about his own son's death. He goes on to say that he tried running away from his sins by using the power of the Titans and tearfully wondered how much better it would have been if he was never born into this world. Zeke promised Ksaver that he would inherit his Beast Titan and use it to take the Founder away from both Eldia and Marley, so he can free the people of the world from the fear of the Titans.
Close to the end of his tenure as the Beast Titan, Ksaver takes Zeke aside and reveals something he did not pass on to the Marleyans: he discovered a way to break the war renouncement vow made by Karl Fritz. He says that in order for it to be broken, a member of the royal family must work in tandem with the inheritor of the Founding Titan in order to make the euthanization plan succeed. Ksaver requests that Zeke find an inheritor of the Founding Titan that will understand their motives and agree to the plan. In 842, Ksaver is chained up to an underground altar and fed to the mindless Titan form of Zeke. Just prior to being consumed, Ksaver closes his eyes and silently tells Zeke that he will always watch over him.
After Zeke Yeager loses his glasses in a battle against Levi Ackerman in the Titan Forest, he remembers his time with Ksaver and then reminds himself that he has a mission to do. Having recalled their history together, Zeke screams for Ksaver to watch him before detonating the Thunder Spear lodged in his stomach.
Ksaver's main occupation in Marley was being a Titan researcher, implying he was experienced in Titan physiology and had a degree of knowledge about Titans.
Ksaver was the former inheritor of the Beast Titan (獣の巨人 Kemono no Kyojin), before transferring this power to Zeke Yeager. Though his Titan form is never shown, he possessed the same regular abilities as Titans and other inheritors of the power of the Titans, such as regeneration. However, Ksaver claimed his Titan was not useful in war.
- Zeke Yeager - Initially watching him from afar, Ksaver formally met Zeke one day after the latter's training session. Over time, he would bond with Zeke and give him guidance and advice when needed. Ksaver cared for Zeke so much that his final thought before dying was to acknowledge Zeke as a surrogate son and that he would always be watching over him.