Refusing her knighthood causes Sirris to go hollow. Once this happens, she can be found in the Pit of Hollows, where she can be killed for the Sunless Talisman. In either case, following her death the Shrine Handmaid will have the Sunless Set for sale.
"Mm, you're an Unkindled, aren't you. I am Sirris, of the Sunless Realms, former servant of the Divinity. Duties we each bear, but one's duty is a solitary affair. I doubt we've much to gain from fraternisation. Blessing of the moon upon your journey."
Second encounter at Firelink Shrine
"Hello again. I have since heard a great deal about you. For one, that you are most gentle of heart. I, too, am bound by duty, but can offer you my sign. I hear that cordial intrusion lays the path to embers. If I can be of help, by all means, do call upon me. Blessing of the moon upon your journey."
"If you should require assistance, use my sign. Blessing of the moon upon your journey."
Assisting with Creighton the Wanderer
"Thank you for your kind assistance. Blessing of the moon upon your journey."
Third encounter at Firelink Shrine, once Creighton has been killed
"Mm, I have not thanked you, for your generous rescue. That Dark Spirit was one of Rosaria's Fingers. Vile bastard offspring who lurk in the darkness. My sworn enemies. Fearsome invaders, to say the least. I would not have made it alone. You have my deepest gratitude. If you require help on your travels, I offer you my sign. Blessing of the moon upon your journey."
Assisting with Holy Knight Hodrick
"I finally found you. Just as I promised, granddad, remember?"
"Good night, granddad..."
Fourth encounter at Firelink Shrine, once Hodrick has been killed
"Oh, there you are. I'm afraid I've involved you in my affairs, over a little promise, at that. My sincerest thanks for your assistance. At last, my grandfather will rest in peace, and I can die. But there is...one last thing. May I take a vow? To serve you, as a knight."
"Yes, of course. I am afraid I have overstepped my bounds. But if you might ever reconsider, and allow me to avow myself to you... Do not hesitate to speak up. Blessing of the moon upon your journey."
Speaking to her after choosing "Refuse"
"Oh, hello again. May I avow myself to you? As a faithful knight, yours and true."
Choosing "Accept her Knightly Vow"
"Oh, I am most grateful. I, Sirris, do hereby serve as your faithful knight. Wherever, whenever, I am needed, and even if all should turn against you... My loyalty shall never waver. Blessing of the moon upon your journey."
"If I may ever be of assistance, call upon me. I am your knight, forever and true. Blessing of the moon upon your journey."
Player joins Rosaria's Fingers
"Mm, I see that you are now a Finger of Rosaria. Your path is wholly different to mine.
Gentle Unkindled, I bid you farewell. If we meet again, it will be as adversaries."
When fighting with her after joining Rosaria's Fingers
"Yes, I see, an errant Finger you've become. Very well. I will cut each Finger from the hand."
At any point in her questline, it is possible to obtain her talisman and set by killing her whenever she appears in Firelink Shrine. Completing her quest beyond defeating Creighton of Mirrah will allow the player access to Hodrick's equipment as well as her assistance in later boss fights.
In previous versions, simply equipping the Rosaria's Fingers covenant item, or using Pale Tongues to access Rosaria's rebirth services would also end Sirris's questline. Similarly, offering Vertebra Shackle to the Mound-makers covenant would also end the questline.
The unique Budding Green Blossom purchased from the Shrine Handmaid after defeating Aldrich has a special description: "Good-bye, dear Grandmother. I will visit Grandfather, in the Pit of Hollows.", which may imply that the handmaid is actually Sirris's grandmother, and possibly Hodrick's wife.