While Bellclaire can certainly provide the health and damage to make her a worthy summon against the Ruin Sentinels, her AI and lack of Poise can turn her into an easy prey for bosses: she may decide to jump down the ledge to chase a Sentinel that has fallen or is approaching from the groundfloor or she can be pushed down by many of their attacks, which would prompt the player to follow her down if they don't want to let her get stunlocked to death in a corner of the room.
She has an easier time against the Dragonriders.
She can use Estus.
Her sign is found in front of the fog gate of the Dragonriders.
Lion Warrior Helm
White Hollow Mage Robe
Creighton's Chain Gloves