There are 3 sets of in-game costumes. Cameo costumes from Tales of Vesperia for Asbel, Cheria, and Sophie. Formal costumes for all. Swimsuit costumes for all. You can get the cameo costumes as soon as you get to Sable Izolle. Both the Swimsuits and Formal costumes are unavailable until you get the shuttle. Before the swimsuits are available you must watch the event at coordinates X: -367, Y: -159. Two costumes require access to the cat village which is available after feeding all the cats all over the world. After going to the cat village, talk to the human man and go to coordinates X332.000 : Y -392.000.
Asbel's Yuri Lowell cameo costume - Awarded for winning the Carta Card minigame on easy without the cpu scoring a single point.
Sophie's Patty cameo costume - Awarded for winning the Carta Card minigame on normal without the cpu scoring a single point.
Cheria's Rita Mordio cameo costume - Awarded for winning the Carta Card minigame on hard without the cpu scoring a single point.
Asbel's Swimsuit costume - Found in a pot in Oul Raye (port town in Strahta)
Asbel's Formal costume - In Barona catacombs—the area where Sophie "died" when Asbel and friends were kids. Examine the altar for a Model Boat. Turn this in to the maid in Lhant Manor for the request.
Sophie's Swimsuit costume - Strahta Cryas ruins. There's an enemy along the West path from the save point.
Sophie's Formal costume - Go to coordinates X334.633 : Y180.499 on the world map, and talk to one of the turtlez (should be the middle one)
Cheria's Swimsuit costume - Examine the right-hand desk behind the item shop owner in Sable Izolle to buy this for 100,000 gald.
Cheria's Formal costume - In Barona castle, one of the rooms has an item needed for a request at Katz village. Turn the item in for her costume.
Hubert's Swimsuit costume - Inside the Rockgagon. Find the watermelon enemy for the costume. You can then exit out the butt.
Hubert's Formal costume - Behind the Strahta Valkines is a chest. Turn the item in the chest in at Katz village.
Malik's Swimsuit costume - At one of the gates to Yu Liberte is a man with a sub event smiley over his head. Talk to him.
Malik's Formal costume - At the end of Bathus Citadel on Fodra.
Pascal's Formal costume - Head East of Oul Raye. Eventually you'll encounter a group of enemies and a scene.
Pascal's Swimsuit costume - First, you need the Amarcian key from a chest in the Bathus Citadel. Just passed the area where you found her formal costume, theres an underground ruin. On the first screen, there's an enemy wearing her Swimsuit. Kill it with fire! (Not literally… <.<)