Gakuto-sama's Daily Wine
[from Volume 1, page 123] A short side comic that answers the question burning on everyone's minds: so how exactly does Gakuto spend his free time? (Note: The title of this comic was changed to "Gackto-sama's Day" in the Del Rey version.)
Intro to Volume 2 and Volume 3
For anybody who needs a brief, quick overview of the story so far.
Original Short Story #1
[from Volume 2, page 13] All of the characters take turns commenting on the release of Volume 2.
Original Short Story #2
[from Volume 3, page 34] Lucia, Hanon, and Rina comment about the release of Volume 3. Lucia and Hanon talk about love, and Rina talks about her abdomen.
Omake Short Manga #1: Gakuto-sama's Extreme Melancholy
An extra manga story included with Volume 3. Gakuto laments the women in his life, and Lucia sings. Plus a myterious flying bra.
Omake Short Manga #2: Private Time
An extra manga story included with Volume 3. Lucia reveals the secrets of how to wash a mermaid's tail, and... Rina sings enka.
Lucia's Beach Side Talk
An extra manga story included with Volume 4. The girls have a steamy conversation; and readers finally get to see (Hanon's imaginary vision of) Mitsuki-sensei without a shirt.
Note: The special stories "Hoshi Furu Umi no Serenade," "Winged Message," and "Soshite Mirai e Super Love Song!" can be found with the regular manga chapters. Why not here? Because I consider these two stories to part of the actual manga storyline, rather than just silly side-jokes like the sections linked above.