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The Reason I Fight (戦いの理由 Tatakai no Riyū?) is the 4th and final chapter of the 13th volume and the 48th chapter overall of the Attack on Titan: Before the Fall manga, written by Ryō Suzukaze and illustrated by Satoshi Shiki.
Sharle goes to see Xavi and while Xavi insists he is there on Gloria's orders, Sharle believes he is there to ensure the Survey Corps fail. Elsewhere, Cardina comes up with a plan for all the trainees to see the vertical maneuvering equipment in action before retaking the aptitude test. The next day, Jorge takes the trainees on a trip to Wall Maria and tells Rosa to find a reason for joining the Corps.
Sharle goes to see Xavi, who invites her to sit down. When asked what is on her mind, Sharle insists that she will not be returning home; Xavi brushes it aside, criticizing their father for not raising her right. He reveals that due to their family name, Sharle has been able to live a comfortable lifestyle in an area starved for resources. Xavi goes on to say that he has a duty to protect his father's company and livelihood of the employees, chiding Sharle for abandoning this duty. He assures Sharle that he will not be going home until the end of his assignment here; until then, she is allowed to do as she pleases. Sharle asks why Xavi is really there, why he is willing to expose himself to the dangers of going on the expedition. Xavi claims it is due to Gloria's orders but Sharle sees right through the lie, believing it is to ensure that Kuklo and the Survey Corps efforts will end in failure. Xavi does not reply, but starts to smile.
During the meeting earlier, Cardina proposes an idea. Due to only five trainees passing the test, he is in favor of a backup unit comprised of twenty five other high scoring trainees that would be classified as a special training class; therefore, the entire group would be subjected to seeing the equipment in use before another round of testing takes place. In the barracks, Jorge meets with the trainees and gives out the results of the test earlier in the day. The five that passed who will be allowed to use the vertical maneuvering equipment are Ivo, Eugen Hufner, Kurz Mauer, Franz Raiffeisen and Hugo. While the others applaud, Rosa is disappointed but glad that two of her friends were able to succeed. However, Jorge reveals that she, Kai and Felix are part of the twenty five backup members who will be eligible in retaking the test in ten days. Jorge informs everyone that he will take those who did not place and instruct them in logistics, scouting and patrolling skills while Kuklo and Cardina will be handling the special training group.
The next day, Carlo tells the trainees there will be no training today and they will instead be taking a trip to Wall Maria. As they travel via wagon, Rosa goes to Jorge and offers to take the reigns; the latter insists she rest like the others but allows her to stay up in front. He remarks that Rosa is just like her mother Maria, and informs Rosa that her mother is known as the "Guardian Deity of Wall Maria." He tells Rosa that he is aware of her mother not wanting Rosa to join the Corps, but Rosa insists that it is not going to stop her. Becoming somber, Jorge informs Rosa that her father Sorum was a subordinate of his, along with Kuklo's father Heath and were both excellent soldiers. Not only that, Jorge tells Rosa that if she wants to convince her mother, she needs to find her own reason for fighting aside from vengeance.