Found in the Church of Yorshka in Irithyll of the Boreal Valley
Plays a role in the Lords of Hollow Questline.
She will be waiting for Anri at the Church of Yorshka regardless of the player having taken Yoel's quest to draw out True Strength and will kill Anri unless killed.
If you go back to Darkmoon Tomb not having killed the Pilgrim Assassin and not being on the Lords of Hollow quest, you will find the its corpse and can pillage Chameleon from it. You can also loot Anri's Straight Sword from the altar.
This NPC is disguised as a statue in the Church of Yorshka next to other statues, to the right of the exit from the bonfire.