I'm kinda busy with Schoolwork and Exams, but I'll help you with this.
So,the steps are:
1) Go to "[Root folder] > data > TH_12.3"
2) Find "select.def"
3) Edit it
How to edit:
1) Download a character you would like to play
2) Extract it in "[Root folder] > chars"
3) Go to that characters' folder*
4) Find its main DEF file and copy the name**
5) Paste the name in the "select.def" file mentioned above
6) Save and play
*For example, my character is Tewi by KON, I extract the folder called "tei" in "[Root folder] > chars", so I go to "chars > tei"
**The main DEF file is usually the same name as its folder, so find "tei.def", so I'm gonna be using in the select DEF file is "tei/tei.def"
Hope this helps~ ^^