Sirris of the Sunless Realms is a pretty cool NPC you’ll encounter in Dark Souls III. She’ll show up in Firelink Shrine after you first encounter Anri and Horace at the Road of Sacrifices bonfire. When you first meet her, she’ll tell you that there’s nothing to be gained from fraternization, and she’ll leave the Shrine. To make her reappear, you have to reach the Old Wolf of Farron bonfire in Farron Keep. Near the elevator, you’ll notice a corpse high up on the wall with an item you can grab.
To get the item, head outside and go around the tower to find some steps you can ascend. Check the walls for an illusory wall you can pass through to get the item, which are ashes you can give to the Shrine Maiden. Head back to Firelink, give the Shrine Maiden the ashes you just found, and then either reload the game or leave the area and come back. Sirris should reappear in Firelink Shrine. Talk to her and exhaust her dialogue once more.
Now, you should be able to be summoned by Sirris at the bridge leading to Irithyll. You have to kill the giant lightning dog on the bridge before you can find her summon sign. It should be in the middle of the bridge. Interact with the sign, and you’ll be summoned to her world to fight the invader Creighton. Defeat him together with her, and then talk to her again at Firelink Shrine. Now, just progress with the story until you kill Aldrich, and then return to the Undead Settlement.
Head towards the boss area where you fought the Curse-Rotted Greatwood and you should see her summon sign outside the area that would ordinarily be blocked off by the boss fog. Interact with the sign and help her kill Holy Knight Hodrick. Once you’ve done this, return to Firelink Shrine and talk to her. She’ll tell you that she’s finally ready to die in peace. When you return to Firelink Shrine again, you’ll find her body near one of the graves outside of the Shrine, and you’ll get the Sunset Talisman and Sunset Shield.
Do note that if you join the Rosaria’s Fingers covenant or even speak to Rosaria at all, Sirris will consider you an enemy, and the questline will be broken.
Be sure to check out our Dark Souls III wiki for more tips and tricks.