This will likely be the last achievement you get in the game, as the final outfit requires you to beat a secret post-game boss. I'm going to list each outfit needed for each character, and do my best to tell you how to get them.
HeroCobblestone's Finest - In the post-game, after you've rebuilt Cobblestone, head to Hero's home and search the wardrobe there.
The Fugitive - Found in a wardrobe in the Gallopolis Church
The Savior of the Sands - Prize for winning the Gallopolis Silver Cup on Easy difficulty
The Drasilian - Crafted in the Fun-Sized Forge. Find the recipe book 'Dress Like a Drasilian' in the ruins of Dundrasil
Daddy's Boy - In Act III, go to The Warrior's Rest in and spend the night. You'll dream of Irwin (this will be the second time). Head back to the Dundrasil Ruins, beat the fight, and the armor will be in a chest
The Parade Participant - Story related, cannot be missed
The Luminary of Legend - Requires the items 'Erdwin's Coronet' and 'Erdwin's Tunic', both found in The Luminary's Trial in Act III
Hotto Stuff - Purchased in Hotto in Act II
The Trodain - Available from the start of the game. Go to Menu -> Misc. -> System Settings -> Claim Special Rewards to receive the Trodain Set
ErikThe Swindler/Spiky Swindler - In Puerto Valor, there is a chef in need of assistance. Helping him will grant you the 'Sweet Stuff for Swindlers' recipe book, from which you can craft this outfit
The Swindler King/Spiky Swindler King - In Act III, once you have the Ultimate Key, head back to the Heliodor Dungeons (can be accessed through the castle) and open the jail cells to find the recipe book 'King of the Swindlers'.
The Corsair/Coarse-Haired Corsair - Requires the Corsair's Cap and Corsair's Coat. The Cap is a drop from the Gyldygga fight in Act II, and the Coat can be acquired in the Nautica shop. (Note: In order to access the Nautica weapon shop, you need to have beaten Alizarin in the story. Once you have, go back to Nautica and talk to the Mermaid Queen to turn back into a fish)
The Pirate King/Prickly Pirate King - Requires the Pirate King's Cap and Pirate King's Coat. Once you're in Act III, head to Lonalulu and solve their problem:
*** Spoiler - click to reveal ***
This will get you the Cap. Once you've rebuilt Cobblestone in Act II, you can buy the coat from Derk's shop
The Stone-Cold Killer - Found on Insula Centralis (Head way North and use Lorelai's Harp to go inland. Find a dragon mount and fly to one of the small islands on the edge to snag the chest)
SerenaThe Hip-Shaker - Buy The Dancer's Costume in the Gallopolis armor shop
The Light Fantastic - In Tickington, once you've finished the quest at Quester's Rest, buy the Divine Dancer's Costume for 52,000 gold
The Dotty Number - Find the 'All Things Nice' recipe book from the masked man near the entrance to Puerto Valor. (Note: in the second half of the game he's found on a balcony over the casino)
The Medal Maniac - Find the Uniforme de l'Academie. There are three of these in the world, but the easiest to obtain is for turning in 25 mini-medals at the academie (Note: You only need one uniform to unlock the outfit for all three ladies)
The Mega Medal Maniac - In Tickington, once you've finished the quest at Quester's Rest, buy the Uniforme Chic de l'Academie for 52,000 gold (Note: You only need one uniform to unlock the outfit for all three ladies)
The Saint - Crafted from the recipe book 'Sacred Styles for Saintly Ladies' which is found in a chest at The Battleground
The Sacred Sorceress - Crafted from the recipe book 'Gifts from the Goddess' which is found on Insula Incognita (due South from The Strand, since there's no freaking names on the map and you can't teleport to them)
The Serenica - Requires the items 'Serenica's Circlet', found in The Sage's Trial, and Serenica's Surplice, found in The Luminary's Trial
Skintight in Scarlet - Awarded for beating the 4th Page in the Altar of the Elusive Age (unlocked in Tickington after clearing all of the other tockle sidequests)
The Mischief-Maker - When you first get to Haven's Above and visit the elder watcher, three chests appear. One of the chests contains this outfit.
Edit: Tenkizuna pointed out that you also need the Ring of Changes for Serena. When completing Drustan's Trials, one of the wish options is something like "I wish I could change Serena's hairstyle. If you choose this option, you get the ring, which allows you to make her hair short again.
VeronicaThe Kitty Cat - Requires the Cat Hat and Cat Suit. The hat can be bought at the Gondolia armor shop, and the suit can be bought from the two brothers in Gondolia (If you're walking in the door, head left to the stall for the first, and up the stairs from the second). Make sure you go back and forth between the two brothers to get the price down.
The Black Cat - In Tickington, once you've finished the quest at Quester's Rest, buy the Black Cat costume for 52,000 gold and the Black Cat Hat for 32,000 gold
The Piglet - Available from the start of the game. Go to Menu -> Misc. -> System Settings - Claim Special Rewards to receive the Baby Boar Set
The Spotty Sorceress - Find the 'All Things Nice' recipe book from the masked man near the entrance to Puerto Valor. (Note: in the second half of the game he's found on a balcony over the casino)
The Medal Maniac - Find the Uniforme de l'Academie. There are three of these in the world, but the easiest to obtain is for turning in 25 mini-medals at the academie (Note: You only need one uniform to unlock the outfit for all three ladies)
The Mega Medal Maniac - In Tickington, once you've finished the quest at Quester's Rest, buy the Uniforme Chic de l'Academie for 52,000 gold (Note: You only need one uniform to unlock the outfit for all three ladies)
The Flapping Fairy - In Act III, head back to Octagonia.
*** Spoiler - click to reveal ***
After you finish that quest, find Sinderella and Whambelina in the casino to obtain this costume (Note: You need to talk to both fighters, each of them has a piece of the costume)
The High Priestess - Crafted using the book 'Eternal Elegance' which can be found in The Sage's Trial
The Goody Two-Shoes - When you first get to Haven's Above and visit the elder watcher, three chests appear. One of the chests contains this outfit.
SylvandoThe Head-Turner - Crafted using the book 'The Dapper Chap', which you get for completing the sidequest 'Skincare for the Fierce and Fabulous' in Octagonia
Even Suaver Sylv - In Act III, head back to Octagonia.
*** Spoiler - click to reveal ***
After this is complete, talk to Golden Boy on the stage to get the recipe book 'Dress to Impress'
The Parade Participant - Story related, cannot be missed
The Parade Master - At the End of Act I there is a side story with Sylvando. By recruiting all possible soldiers of smile, you get this outfit. If you miss it, can be bought at the Octagonia casino in Act II
The Superstar - Crafted from the book 'Glam Gear for Go-Getters' which is the reward for completing the 'Madame Labouche's Life Lesson' sidequest (Found at the mini medal academy)
The Stunner - Crafted using the book 'Glammer Gear for Go-Getters' which is obtained by turning in 90 mini medals
The Don - In Act III, head to Puerto Valor and talk to Don Rodrigo
*** Spoiler - click to reveal ***
After completing this, head back to Don Rodrigo's villa and talk to Servantes to get this outfit
RabThe Sun Lord - Crafted from the book 'Kit Fit for a King' which is obtained by turning in 30 mini medals
The King of the World Tree - Crafted from the book 'What a Wonderful World Tree' which is found in The Sage's Trial
The Sun King - Crafted from the book 'Corking Kit for Kings' which is obtained by completing the third trial at the Wheel of Harma.
The Sun God - A bit of a pain to get. Crafted from the book 'A Guide to Godly Get-Up' which is a reward for beating the final trial in the Wheel of Harma in under 40 moves. If you're having trouble check out this reddit post: https://www.reddit.com/r/dragonquest/comments/9gh6j5/dqxi_wh.... (Not my solution, name of original author not present on Reddit)
The Grandest Master - When you find Rab in Act II on Mt. Pang Lai there is a chest next to him that contains this outfit
JadeJade has the most outfits in the game (gee I wonder why) and the most difficult one to obtain. Jade is the reason this will be the last achievement you get.
The Bunny - Requires the pieces 'Bunny Ears', 'Bunny Suit', and 'Fishnet Stockings'. The ears can be purchased in Gallopolis, but the suit and stockings need to be crafted using the recipe book 'Down the Rabbithole' which can be purchased from the Puerto Valor casino for 500 tokens
Edit: User Mandalore Great pointed out that you can also get the fishnet stockings in Heliodor, in the house just East of the church. You can find them in a wardrobe in said house.
The Hare-Raiser - This is it. This is the one you'll have nightmares about forever. To obtain this outfit, the Hare-Raising Suit, you need to defeat the secret boss 'Timewyrm and Timewyrm's Tail'. In order to unlock this boss, you must complete all of the Tickington side quests, including all the boss fights in the Altar of the Elusive Age.
*** Spoiler - click to reveal ***
This is easily the hardest fight in the game, so be prepared. Have your characters at level 99, get the best equipment you can, bring plenty of Elfin Elixers (Octagonia Casino) and Pep Pops (Rebuilt Cobblestone) and pray as hard as you can to the RNG gods. Good luck.
The Medal Maniac - Find the Uniforme de l'Academie. There are three of these in the world, but the easiest to obtain is for turning in 25 mini-medals at the academie (Note: You only need one uniform to unlock the outfit for all three ladies)
The Mega Medal Maniac - In Tickington, once you've finished the quest at Quester's Rest, buy the Uniforme Chic de l'Academie for 52,000 gold (Note: You only need one uniform to unlock the outfit for all three ladies)
The Swimmer - Purchase the Scandalous Swimsuit at the Octagonia Casino for 70,000 tokens
The Slinky Swimmer - Purchase the Hot Bikini at the Octagonia Casino for 150,000 tokens
The Warrior Princess - Crafted from the book 'Pride of the Valkyries' found in the Fortress of Fear in Act II
The Minerva - Crafted from the book 'The Way of the War Goddess' which is at The Battleground, which you need the Ultimate Key to unlock
The Divine - Buy the Divine Bustier from the same brothers in Gondolia who sold the cat suit. Haggle with them until the price is 20,000 gold (from a nice cool 980,000 to start)
The Xenlon - Crafted from the book 'Shine On, Xenlon' which is awarded to the player for completing the fourth trial at the Wheel of Harma in under 20 moves
The Scarlet Slayer - Purchase the Battle Dress from the shop in Angri-La
HendrickThe General - Crafted from the book 'General Directives', found in the Fortress of Fear
The Field Marshal - Crafted from the book 'Field Manual'. Head back to the Cryptic Crypt near Hotto in Act III after you've obtained the Ultimate Key
The Warrior King - Requires Drustan's Helm and Drustan's Armor. The helm is found in the Disciple's Trial, and the armor is found in the Luminary's Trial
The Heliodorian - Story-related, cannot be missed
The Unswerving Companion - At the start of Act III, after you've regained all your character levels and abilities, head back to Heliodor Castle. You'll find a maid freaking out about some spirit in the courtyard. Follow said spirit through a small questline culminating in a fight with ghost Jasper. Win the fight to get this armor set
One last thing! Getting every outfit for each character is an accolade in-game, which is a great indicator of your progress. This way you already know which characters you have all the outfits for. Happy hunting!
(Edits for formatting and typos)
