Familiar

The Familiar were a race of barely sentient, water-dwelling, hive-minded creatures. PROSE:Dead of Winter (novel)

History
The Familiar popped up quite often throughout Earth's history - most often in anecdotes told by lonely travellers who had been "guided across a treacherous moor" by the supposed spirits of long-dead loved ones or friends that appeared to them. These "living ghosts" were in fact the Familiar, who read could read people's thoughts and use the information to create a near-perfect double of the one they loved most, solely for the altruistic purpose of helping them.

Characteristics
The Familiar were purely benevolent in nature, but impressionable; if they encountered the wrong person with enough influence, they would willingly follow him or her.