I’m Quitting Heroing

Hero Leo Demonheart defeats Demon Queen Echidna but is rejected by humans. Offering his help to Echidna, he is rejected but gains respect from her generals. They secretly hire him to rebuild the army. Leo hopes to learn why Echidna spared humans during the war.

Leo discovers the army's issues stem from injured leaders, leaving only an exhausted Shutina to run things. He takes over replenishing the mana reactor, teasing Shutina. Creating a talisman, he allows anyone to replenish it, reducing Shutina's burden. Impressed, Shutina admits he solved a problem. Leo then ensures supplies arrive safely, worried about Lili’s casual guard mission.

The Dragon-folk supply the Demon Army. Because Lili is immature, Leo is sent to ensure safe supply lines, finding chaos instead. He fakes a snake bite, sending Lili for herbs to help her mature. Despite her brute strength bypassing problems, Lili learns and grows. Shutina is pleased, but Echidna demands to meet the Black Knight Onyx, Leo's disguise.

Leo meets Echidna, disguised, and learns she wants the Philosopher's Stone to stop fighting in the Demon World. She respects Leo for his duty, despite hating him for thwarting her. Leo sees Echidna's good intentions and wants to help. He nearly reveals his identity, but Shutina and Lili interrupt. Mernes is on a journey, and Edward plans to take his own life over losing the war.

While Echidna talks to Edward, Leo trains Mernes, who fears upcoming interviews due to poor social skills. Mernes practices as a disguised waitress, learning basics despite stress and doubts Leo's motives.

Mernes, a half-demon/half-human sold into slavery, joined Echidna's army but was disappointed by her mercy. Leo, over 1000 years old, reveals his true origins and advises Mernes to enjoy life and assures him of his good communication skills.

Leo learns Edvard overtrains his soldiers, not understanding their limits. Leo promises to help him. Using malfunctioning Phantom Nines, Leo assists Edvard, who realizes his mistake and apologizes. Echidna enjoys destroying the Phantoms.

As Black Knight Onyx, Leo plans to reveal his identity to Echidna with Shutina's help. Reading Eibrad's book, a favorite of Echidna's, amuses Leo as he knew Eibrad in Tokyo. Eibrad, an Imp preaching coexistence, warned Leo about his purposeless immortality post-war. He encouraged Leo to find joy in living. Remembering Eibrad’s words, Leo now enjoys life since humans no longer need him.

The Demon World's portal collapses, forcing Echidna's return. Leo reveals the Philosopher's Stone is a black-hole engine and suggests finding the second one. Echidna plans peace to locate a guide. Leo challenges Echidna and the Generals to defeat him and use his heart for peace; if they fail, he will destroy humanity.

Attacking Echidna, Leo claims war gives his life meaning. He fights to destroy everything, tired of protecting humanity. Edvard, Lili, and Mernes are defeated. Echidna uses an ultimate spell, Hero Binding: Anti-Leo, to disable his abilities. Shutina distracts Leo before the spell hits.

Before the spell hits, Leo thanks Echidna and recalls his life. He wanted world danger and created destructive Demon Hearts. Going insane, he destroyed them, met Echidna, and saw her as his successor. She realizes he wanted saving. Echidna keeps him powerless, allowing the Four Generals to kill him.

As Leo lies dying, Echidna almost removes his philosopher’s stone but hires him as an ambassador instead. Leo deletes his protective programming and agrees to help Echidna. He’s happy to quit being a hero. Julietta interrupts, revealing a mountain of work awaits.