Aqua Knight

Aqua Knight (水中騎士, Suichū Kishi) is a manga by Yukito Kishiro.

Plot
Lady Ruliya, a young knight in training, becomes lost during a storm and sinks in her submersible armor. A young boy, Ashika, saves her with the help of his father, Arrabarus. She finds herself on the lighthouse island they guard, far away from civilization. As thanks for his help, Ruliya promises Ashika anything, but she immediately regrets it as Ashika wants to become a knight as her. She is a woman of her word and shows her skill as a knight in fighting Arrabarus, who turns out to be an old aqua knight.

However, the duel is interrupted by the travelling genius Alcantara in his flying fortress. He steals the lighthouse's light sphere, which Ashika calls Niselle and claims is his mother. In the ensuing battle, the lighthouse is destroyed and by some mysterious connection this kills Arrabarus. Ashika is also kidnapped. Tagmec, knight of the Underworld, emerges from the sea to claim the old man, but not before Ruliya promises to rescue the boy and the light. Tagmec informs her she is embarking on a quest of destiny. He grants her Muertogara, a magical blade, for this purpose.

In volume two, more is seen of the eccentric Alcantara, his wondrous inventions and his crew, and see how Ashika endures his captivity. Meanwhile, Ruliya pursues them on her orca mount, but comes across a mysterious knight who invites her to his castle in the middle of the sea. He turns out to be a demon knight who is after both Ruliya and her magic sword. Despite his tricks and charms, Ruliya defeats him. In the process, we[who?] learn more of the sword, the mysterious powers in Marmundo and more about Ruliya's past. Pinoque, a fairy friend from her childhood, returns to her to aid in her quest, and she sets out after Ashika and the light sphere with renewed purpose.

Volume three tells the story of Alcantara, his history in the far-away land of Zygote and his quest in Marmundo. Through a flashback, we learn how he became the greatest genius in Zygote, but was forced to flee because of his unorthodox ideas and dangerous experiments. He escaped Zygote in a zeppelin, taking with him Zykey, a young woman whose mysterious Nil-nil disease prompted him to become the genius he is today. His quest is to cure her of the disease, which makes her disappear gradually.

Lady Ruliya catches up with Alcantara before this, and they duel underwater for the sphere. Ruliya manages to beat Alcantara's masterful swordsmanship in the end, but they emerge from the sea to find that sinister, demonic powers have been plotting to take the sphere for themselves. A great battle against the metal demon ensues, and only with the help of the forces of the Underworld Knight Tagmec can Ruliya finally defeat it with her magic blade. Ashika also denies the demon the sphere by absorbing its light into himself.

Alcantara finds himself seemingly defeated and his inventions in ruins, but finds out that he has found the cure for Zykey's disease by providing her with adventure and a happy life. Zykey reveals her love for Alcantara, and a marriage is arranged. But Alcantara is rescued by his henchmen and flies off in a new zeppelin, followed by a determined Zykey.