The two NPC summons that are available for the Dragonslayer Armour are not appearing. The fan site does not mention that any quest must be completed. One is supposed to be in the Shrine (physical - Sirris), but there are only the main NPCs.
I am Embered when looking for them in the Lothric Castle and it is "The Fire Fades" (GOTY) Edition.
Where are the two NPCs and how to make their summon signs to appear?