From the Super Mario Wiki
Reachers are skeletal ghosts who inhabit the Sunken Ship in the game Super Mario RPG: Legend of the Seven Stars. They resemble large, sad, humanoid skeletons whose bodies are obscured by tombstones. Reachers attack by reaching towards a party member (hence the name) with their long skeletal arms. They can also attack by hurling a large bone (similar to Dry Bones) or by singing a haunting lullaby which will prevent a party member from using magic.
Like most powerful enemies in Super Mario RPG, only one Reacher will appear at a time in battle. Reachers are aided by other ghouls in combat like Greapers.
An unused enemy that appears in the game's programming seems to be a relative of a Reacher, but then with a blue chest. This look-a-like is known as Juju. It was stronger and had a better variety of special attacks such as Silver Bullet and Knock Out, and its Psychopath thought is 'K-9 is after my Bones!'.
For some reason, the original artwork for Reacher has blue eyes, while in the game, it has green eyes. It may have been changed to match the color of its tombstone.
Names in other languages