In the Ringed City DLC, you will come across the Spears of the Church, protecting their Lady Filianore. A small boss fight ensues, in which you kill the guardian Halflight and a player NPC. However, when the game first released, there had to be players who did not fight a Spears of the Church covenant member in that battle. Who participated in the battle if there were no covenant members of the Spears of the Church?
The NPCs wearing the Painting Guardian set are not Halflight. They are, presumably, generic members of the covenant. As far as I know, we don't have any actual lore on them. EDIT my memory failed me but we actually have some info on them, from the Church Guardian Shiv, which they use:
This special throwing blade has a flat tip, and is wielded by guardians of the Church of Filianore.
Guardians of the Church, of whom few remain, watch over Princess Filianore's eternal slumber along with the Spears of the Church.
Halflight himself can be fought if you play offline or if the game cannot find a player to face you. He wears the anticated plain garb set and wields the Frayed Blade, a White Birch Bow and a Knight Shield, and he can use the Divine Spear fragment. You can read a bit of lore about him on the anticated plain garb items description:
Ordinary garb from an ancient land of sorcery. The gold embellishments betray a faint residue of magic, but this clothing was never intended for battle.
When a mission visited the Ringed City long ago, one of its younger missionaries elected to stay behind. It was he who became the last recorded Spear of the Church.
You can see more info as well as a screenshot of him on the wiki.
It doesn't have to be a covenant member, I've been pulled into the fight when my (white) summon sign was down.
However, if the game has trouble summoning a person, the player black crystals out, or you're playing offline, then a NPC will be summoned instead. This NPC has massively increased health over a regular player, and I believe they use a spear and shield.