There are unique NPCs in Dark Souls 3 that only appear in your world to help or hinder your progress. You can't interact with them at all outside of combat. NPCs that invade your world will drop unique weapons and armor sets. Additionally, summoning these NPCs to aid with boss battles can net you new gestures.
This guide details every NPC encounter unique to summoning and invading. If you're looking for general NPCs, Firelink Shrine NPCs, or Covenant NPCs, check out our other Dark Souls 3 guides.
Note that there are spoilers ahead. Not that you care, so let's get on with it.
Not much is known about Lion Knight Albert. You can interact with him twice per playthrough in Dark Souls 3. He can be summoned to help fight Vordt of the Boreal Valley at the High Wall of Lothric. However, he will not help with Dancer of the Boreal Valley if you kill Emma to perform the sequence break.
Later on he's found in the Grand Archives defending the path to the Twin Princes fight alongside a Black Hand rogue and a witch. Killing him here will net you his shield.
Sword Master is the katana-wielding maniac known who massacres new Dark Souls 3 players just outside of Firelink Shrine. If you kill him before facing Vordt of the Boreal Valley, then he can be summoned for that fight. Additionally, you can summon Sword Master for the Champion Gundyr fight in the Untended Graves.
Yellowfinger Heysel is an interesting character. He will invade you on the Road of Sacrifices near the Great Swamp Pyromancy Tome and will drop the Xanthous Crown and Heysel Pick upon death. He will also invade in Farron Keep past the 5 curse frogs.
However, did you know you can summon Yellowfinger Heysel for the Abyss Watchers fight? That's right. To see his summon sign, you must pledge yourself to Rosaria's Fingers and deepen your allegiance to the covenant at least once. You'll find his sign close to the Keep Ruins bonfire on the island with all the slugs near the stone gate. Summoning him will net you the Proper Bow gesture.
Longfinger Kirk will invade a Host of Embers on the bottom floor of the Cathedral of the Deep between the two giant enemies. He drops the Barbed Straight Sword and Spiked Shield on death. Once he's dead, head to Rosaria's Bedchamber and pick up the Armor of Thorns set.
Black Hand Gotthard is a friendly NPC that you can summon throughout your adventures in Lothric. Summoning him for any fight will net you the By My Sword gesture.
His summon sign is found just outside of the Abyss Watchers fight in Farron Keep, and he can later be summoned to fight Pontiff Sulyvahn in Irithyll of the Boreal Valley.
Knight Slayer Tsorig will first invade the player before the wooden bridge in Catacombs of Carthus. Upon death -- or if he kills you -- you learn the My Thanks gesture.
You can later find him as a hostile NPC hiding in the Demon Ruins of Smouldering Lake. Kill him here to receive the Fume Ultra Greatsword, Black Iron Greatshield, and Knight Slayer's Ring. Afterwards, the Shrine Handmaid will even sell you the Black Iron Armor set.
You can also summon him for the fight with the Old Demon King. His summon sign is near the Demon Ruins bonfire.
Great Swamp Cuculus is an NPC summon found just outside the fog gate to the Old Demon King. Summoning her and keeping her alive for the duration of the fight will let you pick up her Spotted Whip and armor set. After the fight, just return to the cage in the Undead Settlement where you first encountered Cornyx to find her equipment.
If you helped Sirris of the Sunless Realms on the grand bridge to Irithyll of the Boreal Valley, then Creighton the Wanderer will see fit to invade you later down the road.
Creighton's invasion triggers in the graveyard near the Church of Yorshka bonfire. Killing him will net you Dragonslayer's Axe. Once he's been dealt with, return to the bridge where you helped Sirris to find his armor set sitting on a fresh corpse.
Londor Pale Shade's allegiance with you totally depends on your allegiance with Yuria. If Yuria is at Firelink Shrine in your game, then Londor Pale Shade can be summoned as help for specific bosses.
You can summon her as help for the Abyss Watchers fight in Farron Keep. Later down the road she can be summoned for Pontiff Sulyvahn, too. If you keep Yuria around for the duration of the game, Londor Pale Shade will even help you beat the final boss of the game, Soul of Cinder. Summoning her for any fight will net you the Duel Bow gesture.
However, if you haven't been following the Dark Sigil questline, then Londor Pale Shade will invade you in Irithyll of the Boreal Valley between the house filled with paintings and the shortcut elevator. Killing her will net you the Manikin Claws.
Alva will invade you on the pathway between Distant Manor and Irithyll Dungeon. Kill him to snag his Murakumo. After he's dead, you can find his armor set deep within Irithyll Dungeon, just outside Karla's cell.
Daughter of Crystal Kriemhild is another mysterious NPC who pops up exclusively as an invader. She invades your world in the Untended Graves where the Cemetery of Ash bonfire would usually be. She drops nothing upon death.
Finally, we've covered every single NPC you can encounter in Dark Souls 3. For the scoop on the rest of the crazy cast of characters, check out the Complete Guides to Firelinke Shrine, NPC Questlines, and Covenants. Heading to NG+? Check out our guide to NG+ Rings. How do you like Dark Souls 3? Chime in below!