Shedinja in depth
The shell that stayed stuck to the tree
The origin is an everyday object in Japan: the 抜け殻 of cicadas, the hollow skin the insect abandons when it emerges from the ground and that remains clinging to the bark for weeks, brown, brittle and split open along the back. The design copies it almost without embellishment, including the dorsal opening that the Pokédex entries turn into superstition.
The Japanese name, ヌケニン, combines “nukeru” — to shed, to come out of something — with “nin”, from ninja, the thread running through the entire line. English repeats the trick with shed and ninja, while Spanish leaves it as is.
The cracks in Wonder Guard
No other species has Wonder Guard naturally; it can only be borrowed through Role Play or Transform. Its list of loopholes has changed from generation to generation, and that list is what truly defines this Pokémon.
In Generations III and IV, typeless damage went straight through it: Revenge, Future Sight and Doom Desire got in without a problem. Fire Fang did too, because of a programming error in those games. From Generation V onwards, that typeless damage became blocked, and only Struggle still gets through every time. What it has never filtered is damage unrelated to typed moves — burn, poison, confusion, recoil, weather and entry hazards — nor one-hit KO moves or moves that bypass the opponent's Abilities.
Comparing it with Spiritomb helps explain why this isn't the same as being resistant. Spiritomb, Ghost and Dark, went two full generations without a single weakness on the type chart until the Fairy type arrived, and in the meantime it took hits normally. Here there's nothing to take: either the attack deals zero, or it deals 1 and the turn is over. The absence of a middle ground is why the species collapses because of a figure that doesn't even come from an attack.
1 HP isn't a stat
Its HP isn't calculated with the usual formula. The game skips it and assigns 1, regardless of level, Nature, IVs or training, making it the only known case of a stat fixed by decree. The consequences keep piling up. Healing items and moves have nothing to heal: Rest appears in its TM list and does absolutely nothing useful. In Generation VIII, it was excluded from Dynamax.
Everything else on its stat sheet is normal. Its 90 Attack is higher than that of many fully evolved Pokémon, and the game gives it moves to use it, including Phantom Force at level 64, which is the strongest move it learns on its own. It learns X-Scissor, Bug Buzz, Giga Drain, Dig and Hyper Beam by TM. None of those details changes anything, because the 1 beside them cancels them all out, and no training, item or Nature can move it even a single point.