Oktavia von Seckendorff is the witch form of Sayaka Miki. She briefly appears at the end of Episode 8 and battles with Kyoko Sakura in Episode 9. She also appears in one of the alternate timelines of Episode 10.
Oktavia appears as a monstrous, immense mermaid wearing heavy knight armor. Her helmet is somewhat reminiscent of an old deep diving helmet. Much like Sayaka herself, she wears a cape (though Oktavia's is torn and ragged) and wields a cutlass (proportionately massive). Her collar resembles a heart, which reflects the reason for Sayaka's fall, the same can be said of her fish tail.
Her barrier is filled with calls for attention - probably to Kyousuke Kamijou. Initially, the outer areas of her Labyrinth are filled with train tracks, perhaps a reflection of the train station where she became a witch. Later on, the entrance area becomes a long corridor filled with concert posters. The center of the barrier appears to be a distorted concert hall, where the seats fill the domed ceiling. Said concert hall is populated with a spectral string orchestra. Listening to it for too long can cause someone to lose their soul.
"The mermaid witch; it is in her nature to fall in love. Looking for the feeling that moved her so long ago, she moves with the entire concert hall. Her fortune only turns under the weight of memories and no longer moves toward the future. Nothing will reach her any longer. She will come to know nothing more. She simply allows no one to disturb her minions' playing."
Abilities and Attacks
Despite Sayaka often being seen as a "weak" magical girl, Oktavia is one of the most formidable witches observed in the anime and manages to cause significant trouble to the powerful and experienced Kyoko, though she still falls far short of powerhouses such as Walpurgis Night and the formidable Kriemhild Gretchen. The witch's weapons are manifested wheels, which she can use to crush enemies. She also wields a huge cutlass, similar to the one she had in her Puella Magi form, but scaled to her enormous size. Oktavia can also use her massive size to grab enemies and crush the life out of them, though this doesn't seem to be very effective considering that she fails to inflict any significant damage on Madoka before Kyouko retaliates. The witch seems to act peaceful as she doesn't attack until an intruder disturbs the play of her familiars.
Kyouko and Madoka attempt to call out to Sayaka while fighting the witch, but this is completely unsuccessful, as the witch is not really Sayaka, only the darkest emotions in Sayaka's heart given physical form. She is eventually defeated when Kyouko Sakura chooses death over a life without Sayaka, and destroys her own Soul Gem (thus killing herself) in a detonation of energy that also destroys Oktavia.
In Puella Magi Madoka Magica Movie: Rebellion Story
WARNING: THIS SECTION OF THE ARTICLE CONTAINS SPOILERS.
Upon becoming part of the Law of the Cycles as Madoka's assistant, Sayaka apparently gained the ability to summon Oktavia Von Seckendorff, who now assists her in battle. Thanks to her gigantic size, the mermaid knight can battle Homulilly on equal footing. Oktavia is now able to summon minions from other Witches, notably Gertrud's Anthonies, she has also been seen wielding a massive version of Kyoko Sakura's spear, symbolizing the deep bond between Sayaka and Kyoko. Sayaka can summon Oktavia only if there is a significant amount of water nearby. In the absense of water she can stab herself to summon Oktavia out of the resulting pool of blood.
- Her wheels could be a reference to the poem "The Wheel of Fate" written by the German poet Karl Siegmund von Seckendorff.
- The name Oktavia refers to the word octave (the eight interval of a scale in music). This name reflects Sayaka's attachment to the musician Kyosuke.
- Sayaka's story closely mirrors the original, tragic version of The Little Mermaid, who makes a deal with a sea witch to be able to court the prince she loves, but said prince ends up choosing someone else over her. As per her contract with the witch the mermaid dies, turning into foam. Oktavia's appearance reflects that fact.