Reason to Fight

Reason to Fight (戦う理由) is the twelfth episode of Season 2 and the thirty-seventh of overall episode The Rising of the Shield Hero anime.

Synopsis
The Wave appears before its scheduled time. Kizuna’s group is teleported to it while Naofumi and his party head for Kyo’s lab on Ethnobalt’s ship. There, they see the true depths of Kyo’s wickedness...

Plot
Naofumi and Kyo narrate flashbacks of their respective normal lives in our human world, feeling how boring and tedious they were, before they both were summoned into the current world. Time cuts back to present, in Kyo's laboratory, the malevolent Book Hero creating a Catastrophe Wave by himself, which starts instantly and causes Glass' party to teleport to the battle site.

Ethnobalt appears in his Vassal Boat to take Naofumi's party to the battle site. Yomogi explains that Kyo used the Spirit Tortoise's stolen power to generate the Wave. Naofumi almost punches her out of fury, but she pledges her allegiance to Naofumi on Kizuna's behalf. Ethnobalt takes them to Kyo's hideout. Kizuna, Glass, Therese and L'Arc are already occupied facing countless hordes of Hindi mythology monsters. Naofumi's party reaches Kyo's fortress, and Ethnobalt departs to help Kizuna's party. Once inside the enemy laboratory, they are faced with Kazuki's trio of bodyguard girls, all of them mutated into bizarre Chimeras resembling a mixture of human and Guardian Beast and holding the same parasitic sword which Yomogi used before. Yomogi tries to dissuade them, but Raphtalia's jewel, which was given to her by Therese, rapidly neutralizes them. Naofumi realizes that the parasitic sword is about to self-destruct, and summons several of his Shields to encase and protect his party against the massive explosion.

Kyo himself appears to mock Naofumi, and aside him is Kazuki - or at least what seems to be him - holding a mirror. Yomogi explains that Kazuki is actually dead, his body being just a reanimated homunculus containing the soul of Albert, the Mirror Hero. Kyo leave letting the reanimated Kazuki to kill them all, but Raphtalia hears Albert begging to be freed (which indicates the Vassal Weapons are somehow resonating with each other). Raphtalia, Filo, Raph-chan, and Yomogi stay behind to take Kazuki on while Naofumi and Rishia go after Kyo. The duo soon reaches Kyo in another room which houses all the power stolen from the Spirit Tortoise, but since the Guardian Beast has died along with Ost Horai, he intends to return to Raphtalia's world to collect more power from "some giant bird", which deeply enrages Naofumi.

Naofumi's ire activates the dangerous Wrath Shield, much to Kyo's insane delight. The Book Hero revels on the grief caused on Naofumi due to separating him from Raphtalia and Filo, as well as Ost's death. As he prepares to launch his Blood Sacrifice against Kyo, the Shield's dark influence on Naofumi awakens the Dragon Emperor, who completely takes Naofumi over, while Kyo keeps mocking him for treating his friends as slaves. Rishia tries to calm him down, but Naofumi strangles her, claiming she's just as useless and weak as Raphtalia and Filo were before and shall be killed first. Rishia, however, feels comfortable with Naofumi's coarse words, managing to bring him back to sanity and reassuring that his "slaves" are indeed his treasured friends, Raphtalia, in the meantime, manages to destroy the Kazuki homunculus and free Albert's soul, while Rishia manages to temporarily subdue the Dragon's dark influence of Naofumi.

This pisses Kyo off, who then goes to fight personally. Rishia rapidly reaches and neutralizes Kyo at sword-point, who then launches the Spirit Tortoise's gravity power against her. Rishia once again calls Kyo weak for relying on powers from another ones. The Dragon tries to influence Naofumi again, but Ost's spirit reminds him of his real task and his friends, repelling the Dragon Emperor completely. Raphtalia and Filo are shown next to Naofumi, giving him power. Kyo then fires a dark energy beam, which Naofumi counterattacks with a light energy beam. Kyo sees Naofumi wielding the Vassal Mirror, which combines with the Legendary Shield, and fires one last energy beam against him. Naofumi, with all his friends at his side, tells that he does fight for himself in his own free will. Kyo is destroyed, and the Calamity Wave is effectively terminated. Ost's spirit bids them farewell, and Ethnobalt appears.

Kyo is dead, and Yomogi grabs his dropped Book Vassal Weapon. Now the world is in peace for the time being, for there's one month until the next Wave. The Shield's interface shows that the Spirit Tortoise's power has been fully retrieved and within one minute they'll all teleport back to Raphtalia's world, much to Kizuna's sadness. Back home, Naofumi is commended with the title of Viscount by Queen Mirellia herself, and he knew that while the Wave struck Glass's world, another one simultaneously befell Melromarc, but the other three Heroes managed to fight and defeat it. Naofumi's voiceover shows Princess Melty, Beloukas, Granny, Eclair, Ake, Sadeena, Erhard and even Keel, all training hard for the incoming Wave against the Phoenix, the next Guardian Beast to be faced. Naofumi reassures his vow to fight aside his friends... as the Shield Hero!

Light Novel & Manga Differences

 * Tsugumi and the rest of the Trash II (Kazuki) group attacked and were captured after attempting to take over the dragon hourglass at L'Arc's castle in the light novel and manga. Here, Kizuna was able to extract the guardian-beast parts. In the anime, they were defeated defending Kyo's lab.
 * Kyo now in his soul heads back down the shaft to acquired a new body, but Naofumi destroyed it leaving Kyo as a monstrous spirit for the Hunting Hero to finish him off, but in the anime he's already dead in the explosion.

Trivia
Information to be added later on.