Underworld soldiers born to fulfill just one simple task: defending their allies. Nearly all ranged attacks are nullified by their impervious shields. Shildeens have a skill for organization rare among the Underworld Army.
Shildeen is likely to be a pun on the word "shield."
Shildeen seems to resemble the Kappa, a mythical creature from Japanese folklore.
It is possible to be inside the shield if the Shildeen spawns next to Pit.
The way Shildeens become stunned after their shields break is similar to the effects of players' shields breaking in Super Smash Bros. Melee and Super Smash Bros. Brawl.