Comments posted to our Dark Souls 3 Wiki
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I think that you should add something like this:

"If you join Fingers of Rosaria after Sirris pledged loyalty to you she will appear in firelink shrine to warn you of her becoming an adversarie of yours, you can kill her here as a last chance of getting the sunless talisman and her armor set"

I'm not sure if she vanishes out of your world after that if you don´t kill her or if she hollows in the normal location, but i would bet for the first one.
I was fighting her in the Firelink Shrine (with her fighting back) and after she self-healed two or three times with her miracles, she just vanished out the world mid-fight when I next got her around half-health. Is she now just gone completely from this cycle and I have to wait until NG+ to attempt to fight her again?
Followup: Turns out, regardless of her just fleeing, she left her armor with the shrinemaiden to buy. Which was the whole point. Now I have her armor and the cloaking ring. Woo.
Further follow-up: her Sunless Talisman appeared on the spot she usually sits after I reloaded the area. So I guess instead of vanishing out of the world, maybe she fell through the world and died?
"Hi, Sirris"
[enters boss\npc fight, gets one shot]
"Bye Sirris"
I talked to her in firelink shrine, she gave me the gesture and dissapeared. I gave the ashes to the shrine-maid and there was sirri again, everything seemed cool. But when I went to fight the Abyss Watchers there was no summon sign, only Gotthards...