Shedinja’s hard body doesn’t move – not even a twitch. In fact, its body appears to be merely a hollow shell. It is believed that this Pokémon will steal the spirit of anyone peering into its hollow body from its back.
– Pokemon Ruby Pokedex
Shedinja is a very unique Pokemon. First of all, its evolution mechanic is unique; a Shedinja is obtained when a Nincada evolves into a Ninjask with an empty slot in the party (and an extra Pokeball in the bag). Its typing of Bug/Ghost is unique to itself. Additionally, its ability Wonder Guard is unique to Shedinja, and it cannot be Skill Swapped or copied by Role Play or be turned into Mummy (though Trace can work on it). Shedinja needs Wonder Guard because of its unique 1 HP stat, which cannot be altered by IVs or EVs.
Because of the existence of Wonder Guard, which grants immunity to the Pokemon by any moves that aren’t Super Effective, people have hacked Wonder Guard onto Pokemon that have no type weaknesses, like Sableye and Spiritomb. However, Wonder Guard doesn’t protect the Pokemon from non-damaging moves, such as entry hazards, weather effects, or status conditions that can easy take out Shedinja’s 1 HP.
Shedinja’s Pokedex entry is rather ominous. It’s basically a cast off shell that gained consciousness. But it also says it steals the spirit of anyone peering into its hollow body from the back, and when you battle with a Shedinja, it has its back to you, so does that mean Shedinja steals its trainer’s soul? Hmm… Also, Shedinja doesn’t have eyes, it has eye holes where Nincada’s eyes used to be…
Aside from its 1 HP, Shedinja has decent Attack. Its Defense and Sp Defense stats are only used if it was Baton Passed a Substitute. Because of its 1 HP, Shedinja has the lowest base stat total of all evolved Pokemon. Its 1 HP is also the lowest a stat can be.