As the leader of the Yōma, Shin is Kagura's primary target, and according to Sayuri it is the entity responsible for a majority of disasters in the Senran Kagura Universe. Few are aware of Shin's existence though, even among the shinobi that know of the Yōma.
Shin has yet to be directly encountered, but when it is spoken of during the Kagura Millennium Festival, it is vaguely shown as what appears to be a giant baby in a crib, still growing.
Shin is the oldest of the Yōma, as it is the original Yōma. In theory, it's destruction could bring about the end of all Yōma.
There appears to be only one Shin across the timelines, and it has resulted in the destruction of alternate Earths in the past. The island where the Kagura Millennium Festival takes place is actually on one such Earth, where nothing outside of the island remains except for water.
The only way to actually fight Shin is to first lure them into the world's own space-time using a very powerful Shinobi Barrier. Blood must be spilled within in order to attract it.
However, as of yet, nobody is prepared to take Shin on should they arrive.
The Peach Beach Splash event is held as a secret way of reinforcing the barriers keeping Shin away for just a little longer to give the shinobi of the world more time to prepare.
- Daidōji managed to punch her way into Shin's realm, but was repelled by a single finger's movement, namely its pinky.