Below is a list of all the costumes for Serena in Dragon Quest XI S.
The Hip Shaker
The first costume for Serena will be pretty easy to acquire, as you can simply purchase it in Gallopolis, upon first visiting the town.
The Dotty Number
This costume will require you to get a recipe (All Good Things), which is gotten by talking to a man in red near the entrance in Puerto Valor (can be done on your first visit). You simply need to speak with the NPC, as you don’t have to do anything else. You will need Faerie Fluff (x2), Buzzberries, Waveweed (x2), and a Crimson Coral to forge both pieces.
The Medal Maniac
The Uniforme de l’Academie is a piece of armor that can be equipped to any of the female party members. There are three of these in the game, although in the Definitive Edition, you only need one in order to access the costume on all three characters. In the original version of the game, you will need to equip them on each character to trigger the costume. You get one for turning in 25 Mini Medals, another is behind the Magic Key door in the Mini Medal Academy, and the third is a reward for completing Quest 20 - The Agony and the Ecstasy.
This is another costume that requires getting a recipe first, called Sacred Styles for Saintly Ladies. The recipe is found in a chest on B9 of The Battleground. In order to forge both items needed for the costume, you will require a Molten Globules, Savvy Sapphire (x2), Brighten Rock (x2), Sparkly Sap, Colourful Cocoon (x2), Pale Pearl (x3), Mirrorstone (x2), and a Sainted Soma.
This costume is exclusive to the Definitive Edition version of the game. Once you get to the final part of the game, you will go to Havens Above and loot the Ultimate Key from a chest. There will be two other chests in the same area and one of them will contain the Little Devil’s Dress piece of armor, for this costume.
The Sacred Sorceress
You will need to find a recipe before being able to obtain this costume. The recipe, Gifts from the Goddess, is behind the Ultimate Key door on Insula Incognita, meaning you won’t be able to get it until the final act of the game. In order to forge both pieces, you will need the following items: Slime Crown, Savvy Sapphire (x2), Brighten Rock (x3), Sainted Soma, Technicolour Dreamcloth (x2), Pale Pearl (x3), Cumulonimbough (x2), and a Mirrorstone.
The Mega Medal Maniac
This costume is exclusive to the Definitive Edition version of the game. The costume is shared across all three female party members, as you only need one of the armors in order to unlock the costume. As you make progress in Tickington, you will unlock an area called Whealbrook. Complete the quest there, then go to Quester’s Rest afterwards, speaking to Sellma behind the counter on the right, to purchase the Uniforme Chic de l’Academie armor.
The Light Fantastic
This costume is exclusive to the Definitive Edition version of the game. It is bought from Sellma in Quester’s Rest in Tickington, so look at the previous costume to see instructions on how to get there.
There are two pieces of equipment needed for this, Serenica’s Circlet and Serenica’s Surplice, both found in different locations. The Circlet is found in the Sage’s Trial - Fierce Forest, while the Surplice is found in the Luminary’s Trial - Citadel of Spite.
Ring of Changes
While this isn’t technically a normal costume, like all of the others, it is still counted and needed for the Dedicated Follower of Fashion trophy/achievement. To get the Ring of Changes, you will need to reach the Vault as part of Drustan’s Trials and defeat one of the bosses. After doing this, you will be able to pick from one of five wishes. One of the five allows you to change Serena’s hairstyle, making it shorter like it was in the middle part of the game. Note you will need to actually equip the ring in an accessory slot, as it doesn’t register as a separate costume in the Definitive Edition.
Skintight in Scarlet
This costume is exclusive to the Definitive Edition version of the game. In order to get the piece of equipment for this one, you will need to do all of the Tickington altars in the main room. Upon doing this, another room will open up, with one final altar, called the Hall of Remembrance. These will pit you against a bunch of bosses on four different levels. Complete the boss battles on Level 3 and your reward will be the Crimson Catsuit, netting you this costume.