How to Combine Costumes
by cyllya

Make sure you have the costumes you want to combine. (This is a shop screen.) You can't use a costume in a combo while you're wearing it. -_^ Take it off first.
Talk to the combination person. (Who it is depends on the Time Period)
Select option #1

 It takes you to a screen with a list of the costumes you can use to combine. (If a certain costume hasn't been mastered by at least one character, it won't be on the list.) Select the first costume you want in the combo. I'm selecting Minarai Kenshi (Squire). After selecting the first Squire, it shows it at the top. Select the second costume, which is another Squire in this example....
 It switches to a new list after selecting the second costume. It now shows the all possible combinations of the two costumes you selected. For Squire + Squire, there's Kenshi (Knight), Kakutouka (Fighter), Mahou Tsukai (Magician), Healer, and Hunter. The last four can be bought at the shop, so don't use the combo for them. Get a Kenshi, and note that the combo will cost you 175 Gald. 

Make a note of the characters' sprites up there. If the green circle is flashing over them, they'll be able to wear the costume your curser is over. If there's a red X, then the character hasn't met the costume's requirements and won't be able to wear it. The Kenshi requires that the Minarai Kenshi has been mastered and the character has 4 L-EXP. (See below for L-EXP info.) Say Mel hasn't mastered the Minarai Kenshi; she won't be able to wear the Kenshi costume.

If the character doesn't have anything over him/her, and his/her eyes are close (like Kururu always is), then the character won't be able to wear the costume no matter what. (Dio isn't going to wear the Priestess costume. -_^)

It takes you to a new screen after selecting the combo, which shows Dio's stats with the combo costume compared to whatever he's already wearing. Push A to confirm the purchase of the combo, or B to decline. (Select "Mel o miru" to see the combo costume's effect on Mel's stats.)

If the character has a red X, it'll tell you here what requirements are needed for the costume.

The stats on the right are:

You're done!

L-EXP is how I abbreviate Life Experience (jinseikeiken). Life Experience is the number of costumes the character has mastered. To see how much L-EXP the character has, go into his/her status screen. Look for the number on the right side below his/her picture, above the CP. In the Narikiri Zukan (or in the screens above where it shows Mel and Dio's stats), the L-EXP is the number at the bottom.