TLDR; https://gtm.steamproxy.vip/app/867490/discussions/0/2572002906843774400/

Literally this.

Go to your steam game library, right click on the Remyadry game and click on 'Properties'


Afterwards, click on 'Local Files' tab, and click on 'Browse Local Files...'


Your file manager should pop-up like this...
(Basically click on the 'custom' folder from here on)


You should see a 2 folders in particular from now on, 'chara_s' and 'chara_t'.


'chara_s' folder stores all the tab-like potrait images (those character tabs you see on the right side), then you have 'chara_t' that stores all the large potrait of the characters.
Also, you should have 2 templates in a photoshop file, as the namesake suggests, those are tempates for you to work on top of if you need to.

From here on, you should be largely safe from following the templates.
If you do not own Photoshop, you could use Pixlr or GIMP editor instead.

Just a few notes if you are encountering some problems:

• Small character potraits should only be in JPEG format! (As of the date of this post) Using PNG images does not work.
  
• Large character potraits should only be in PNG format, using JPEG causes the game to ignore it or display a white background. (Use alpha layering)
  
• Do keep the potrait sizes as shown in the templates. (You can scale down or resize images using Photoshop or some online softwares)

Also, do note that some of the characters will be obtained while playing through the game.
(Hopefully someone will upload a list of those characters)