Steelheart Ellie is a white phantom appearing in all three of The Lost Crowns DLCs.
She is available for summoning in the fight with Elana, the Squalid Queen. Her summoning sign can be found just outside the boss arena.
She is available for summoning in the fight with the Fume Knight. Her summoning sign can be found near the Lowermost Floor bonfire area before the boss arena.
She is available for summoning in the fight with Aava, the King's Pet. Her summoning sign can be found outside the fog gate, once the Eye of the Priestess is obtained.
She fights solely using dual Caestus. Overall, her usefulness does not go beyond her ability to tank attacks, as her damage output is rather insignificant. She wears the Black Witch Veil, Agdayne's Black Robe, Leather Boots and Tattered Cloth Manchettes.
In Crown of the Ivory King, she'll also use these pyromancies: Great Fireball, Great Chaos Fireball, Firestorm and Flame Swathe. She will also use Estus whenever she has the time to recover.