Tom Ksaver was an Eldian researcher, a member of the Warrior Unit, and the previous inheritor of the Beast Titan. He became the mentor, father figure, and closest friend of Zeke Yeager, shaping both his ideals and his ultimate plan for the future of Eldians. 

In his human form, Ksaver had a round face, short dark hair, and wore glasses, which Zeke would later adopt in memory of him. He usually dressed in the standard Marleyan Warrior uniform, marked with a red Eldian armband to show his status as both an Eldian and an honorary Marleyan. His overall look was modest and scholarly, reflecting more of a researcher than a soldier.

In his Beast Titan form, Ksaver’s Titan resembled a bighorn sheep. Standing at 17 meters tall, this Titan had curved horns, a long muzzle, a beastlike nose, and light-colored fur on its head, neck, and upper body. Unlike other Beast Titans who focused on combat, Ksaver used his Titan mainly for research purposes, preferring observation over violence.

Ksaver was known for his kind, easygoing, and intelligent nature. He had an inquisitive mind, joining the Titan Biology Research Society to study the mysteries of Titans. To Zeke, he was a comforting and supportive figure, often spending time with him by playing catch and offering guidance.

However, beneath this warmth, Ksaver carried deep personal pain and self-loathing. He had once married a Marleyan woman by hiding his Eldian identity, but when she discovered the truth, she killed herself and their son. This tragedy left Ksaver with profound guilt and suicidal tendencies, leading him to accept becoming a Warrior — seeing it as a slow form of suicide. Despite his despair, he still cared deeply for Zeke, offering him both affection and wisdom.

• Marriage and Tragedy: Ksaver married a Marleyan woman by hiding his Eldian armband. When his wife found out he was Eldian, she committed suicide, taking their son’s life as well. This devastated Ksaver, leaving him emotionally broken.
  
  

• Becoming a Warrior: Following this tragedy, he volunteered to join the Warrior Unit and inherited the Beast Titan. Unlike other Warriors, he did not aspire to glory or honor — instead, he saw his service as a path to death.
  
  

• Researcher: Ksaver dedicated his life to studying Titans as a scientist, becoming an expert in Titan biology. His main discovery was that the Founding Titan could alter the body composition of Subjects of Ymir, a revelation that inspired Zeke’s Euthanization Plan.
  
  

• Mentor to Zeke: Ksaver met Zeke Yeager during his childhood and noticed his struggles. Instead of pushing him harshly like his parents, Ksaver encouraged Zeke with kindness, often playing catch with him to ease his loneliness. He eventually advised Zeke to betray his parents to save himself, an action that Zeke carried out.
  
  

• Influence on the Euthanization Plan: Ksaver revealed to Zeke how to overcome Karl Fritz’s vow — by combining the Founding Titan with someone of royal blood. Together, they formulated the Euthanization Plan, aiming to end Eldians’ suffering by ensuring no more Subjects of Ymir would be born.
  
  

• Death: In the year 842, Ksaver’s time as the Beast Titan came to an end. He was eaten by Zeke Yeager, passing the Beast Titan’s power to him. His legacy, however, lived on in Zeke’s ideals and his enduring emotional influence.