Basically, my question if is it possible to summon a spectator as a bodyguard. But let me first explain it:
Monster Manual p. 31 says, about Spectators:
A spectator is a lesser beholder that is summoned from another plane of existence by a magical ritual [...].
Magical Guardians. A summoned spectator guards a location or a treasure of its summoner's choice for 101 years, allowing no creature but its summoner to enter the area or access the item, unless the summoner instructed otherwise. If the item is stolen or destroyed before the years have all passed, a summoned spectator vanishes. It otherwise never abandons its post.
Emphasis not mine.
So, imagine that a character knows that "magical ritual" and summons a spectator. (Note that I'm not asking how to summon it, for the sake of the question the NPC already know and can, or, if it is a player, he knows the proper ritual/spell to do it.)
For example, a wizard from Lost Mine of Phandelver, per p. 48 of the Starter Set adventure book):
The monster that guards this room is a spectator. One of the human wizards who worked in the Forge of Spells summoned the creature to guard the magic items created and stored here.
Imagine that instead of protecting the room the wizard wanted to use this spectator as a guard, could he do that?
In other words, can the treasure or item to guard by the spectator be an alive creature (like the summoner)?
So, the spectator will always follow its summoner to protect him.
In the case that it isn't possible, can the treasure or item to guard by the spectator be a moving object (like a necklace)?
So, instead of guarding the character, the spectator follows the character's necklace to protect it from anyone (less the summoner and friends) who dares to take closer and try to steal it.