Narikiri dolls are available in FUTURE ARC ONLY.
Narikiri dolls are used to turn one of your party members into a completely different party member complete with that characters costumes, artes, BC's etc everything except their stats. Lets say you want to have 4 Maliks on a team. You make 3 Narikiri Malik dolls and you can turn 3 other party members into Malik. These party members stats will stay the same, but play as Malik with all of his abilities from titles. The cloned characters will all wear the real Maliks current costume.
In F arc, there is an NPC on Telos Astue who will "Role-play" the characters for you. Basically, this means that he'll give your chosen character the title needed to become another party member, but you must first dualize yourself the Narikiri doll for the character you want your chosen character to become.
Credit for the following information goes to AzureKite01:
Asbel = Corrupted Edge + Everlight (To make everlight you need Luminous Core+ Perilous liquid)
Sophie = Mythical Scale + Giants Shield
Hubert = Killer Fang + Godwood
Cheria = Alluring Feather + Needle of Extinction
Malik = Arcane Gear + Meteorite
Pascal = Fantabulous Fur + Classified Seed
Richard = Incredible Bone + Demons claw
Locations credited to Dimension Door
Luminous Core drops from Hoploid, Evocatoid, Auxilioid (All from Eleth Research Laboratory)
Perilous Liquid drops from Luminescent Roper (Depths of Lastalia Shaft), Paion Slime (Depths of Lastalia Shaft)
Corrupted Edge drops from Sigma Enemies (Lastalia Shaft)
Mythical Scale drops from Sand Drake (Strahta Desert Ruins), Core Drake (Lastalia Shaft)
Giants Shield drops from Natural Star (Fendel Glacier Ruins)
Killer Fang drops from Labyrinth Lizard (Fendel Glacier Ruins), Goliath Bat (Gustworks Ruins), Legendary Wyvern (The field outside Telos Astue)
Godwood drops from Sky Treant (Strahta Desert Ruins), Ruins Goblin (Fendel Glacier Ruins)
Alluring Feather drops from Hellstorm Harpy (Lastalia Shaft), Radiant Eagle (Arcadia Garden)
Needle of Extinction drops from Braid Spider (Strahta Desert Ruins)
Arcane Gear drops from Wailing Golem, Platebreaker (All from Eleth Research Lab)
Meteorite drops from Blitzer Bear (Arcadia Garden)
Fantabulous fur drops from Horkew Kamuy, Ovinnik (Fendel Glacier Ruins)
Classified Seed drops from Midnight Shade (Strahta Desert Ruins)
Incredible Bone drops from Terrane Boar (Lastalia Shaft)
Demon's claw drops from Egregoroi (Arcadia Garden)
Actually, most of the above monsters are fought in the Arcadia Gardens or the Lastalia Shaft, though Arcane Gears are only in the Eleth Research Laboratory.