Umi no Yami, Tsuki no Kage (The Darkness Of The Sea, The Shadow Of The Moon) is a manga created by Chie Shinohara.
The twins Ruka and Rumi Kobayakawa are identical in their looks, their likes, and dislikes. Unfortunately, this includes their preference in Katsuyuki Touma, who shakes things up by confessing his love to Ruka. With Rumi encouraging her to accept his confession, Ruka plans to do just that on Monday.
On Sunday, they head off on a seaside trip with their friends. A sudden downpour leads to them seeking shelter in an odd cave that they are told was an old tomb. Everyone notes an odd stench, only for a wall to crumble and release the inner workings of the tomb. Rushing out for fresh air, each of the girls suddenly collapses onto the ground.
Rumi and Ruka are the only ones that survived the event, with their friends having died from what was noted as asphyxiation. And not only that, whatever happened has caused Rumi to change. She tells her twin that she won't give up on Katsuyuki and refuses to simply hand him over to Ruka.
The manga contains examples of the following Tropes:
- Curse of the Pharaoh: Discussed, as the events start after the twins enter an old tomb. The curse of Tutankhamen is even mentioned by name.
- Family Theme Naming: The Kobayakawa household has three daughters. The twins are called Rumi and Ruka, and their older sister is called Ruiko, retaining the same kanji (流) in all three names.
- Gotta Catch 'Em All: Partway through the series, the twins need to collect the five pieces of paper containing the prescription of the medicine that could cure or re-infect people.
- Hope Spot: The pre-climax focuses on the twins receiving a potential cure for the viral infection, though the doctor notes ahead of time that it might not work perfectly. The cure doesn't work, leaving both of them with their powers intact, and Rumi loses all hope of containing her hatred much longer.
- Identical Twin ID Tag: Averted throughout the series. Identical twins are identical, with it even mentioned that they have the same type of voice.
- Identical Twin Mistake: Happens frequently with Rumi and Ruka. Although Katsuyuki can tell them apart on sight.
- It Always Rains at Funerals: It rains when Katsuyuki's family gets buried.
- Light Feminine and Dark Feminine: Despite being twins, Rumi and Ruka end up getting differentiated in this way as the series continues. Ruka is the light feminine, the protagonist and Rumi is the dark feminine, as the antagonist. Especially noticeable as Rumi becomes more known with wearing darker clothing, while Ruka remains in light colors.
- Lunacy: The odd abilities Rumi and Ruka gained after entering the tomb gain and lose their powers with the moon's phases. They are at their strongest on full moon nights.
- Sibling Triangle: Rumi and Ruka both have feelings for Katsuyuki, though the series begins with him confessing to Ruka. One of the major conflicts is Rumi trying to separate the two and become his girlfriend.
- Spanner in the Works: Katsuyuki inadverdently got himself protected from getting infected and controlled by Rumi. Since he kissed Ruka first and got her 'infection', this gave him the anti-bodies that kept him safe from Rumi's strain.
- Theme Twin Naming: The twins Rumi and Ruka have an alliteration theme. They also share the first kanji, while Rumi has the kanji for water (水) and Ruka the kanji for wind (風).
- Title Drop: The final page of the manga drops the title of the series, with an implication of which twin each half of the title referred to.The darkness of the sea reflects the bright light, and the shadow of the moon is disappearing
- Total Eclipse of the Plot: The final climax takes place during a full moon night and when a total lunar eclipse occurs.
- Twin Switch: Performed repeatedly throughout the series, with one twin pretending to be the other.
- The Virus: The cause of the twins gaining odd powers comes from an infection they gained from the bacteria located in the tomb. Anyone who gets Rumi's blood into their own bloodstream, usually by her biting a person's lips and licking the wound, ends up being under her control. Alternatively, Ruka's particular strain serves as a vaccine to Rumi's strain, making anyone that gets her blood into their bloodstream immune to possession.
