Sidekick/companion

Sidekicks and companions can accompany heroes during their nocturnal exploits, with at most one of each at any given time.

Companions

Accompany the hero for a certain amount of time.
They can be summoned by either using an innate skill, using a summoning item, or acquiring them at a specific location.
If a sidekick is already active, its lifespan can usually be lengthened by repeating the summoning action.
They can be dismissed from the sidekick details page in your Hideout or from the side panel.

Sidekicks

Only available after a certain step in the level 9 quest.
Each of them needs to be enabled once, and from then on they can be selected from the Hideout.
Once selected, a sidekick will follow the hero for as long as necessary, until it is dismissed (from the Hideout as well).
Sidekicks' power scales with the hero's level, until some cap is reached.

Sidekick modifiers

All sidekicks carry some benefit, be it damaging opponent, increasing chips drop etc.
The effectiveness of sidekicks can be improved using effects and equipment.
In addition, judo kid gloves‎ can be equipped, granting the sidekicks additional capabilities.

Notes

• See also Sidekick/companion (by source)
• Sidekicks cannot attack frusion enemies. Instead you get the message:

Your sidekick can't see your opponent, and just tries to stay out of the way.

• Prior to November 27 2014, sidekicks and companions were a single mechanic; as such, only one of either could accompany the hero, and things which improved sidekick effectiveness also affected companions.