From LSWiki

This charm summons a moderately-powerful spirit of elemental fire. These are considered followers, and will provide additional damage in combat; also they provide protection by succoring the consort from enemies. They can be kept, and use the "Fire Creatures" keep capacity. They do not accept commands. Fire spirits fly and emit light.

With higher pyromancy skill, multiple fire spirits can be summoned.

Usage

concentrate on summoning a fire spirit