One of the first things you must do in Astral Chain is to create a character, though it is somewhat limited in what you can pick. You may feel like these choices are locked in for awhile, but eventually you are able to indeed change some of these options a little later and this guide will detail how you can do just that.
To change your character design in Astral Chain, you must visit the Locker Room, which can be chosen from the fast travel list. Once you are there, walk up to the first row of lockers that you see and press A to open the locker.
Within the locker, you can make multiple edits, including Hair Style and Eye/Skin Color as you did at the beginning. In addition, you can then adjust Equipment as well, which includes your jacket, pouches, head accessories, ear accessories, and much more. These are ones that you collect in the game itself, so what you can equip all depends on what you have found thus far.
You can also change your costume, though you very possibly will only have your Ark Police Gear available at this point. You will have to unlock these in the game separately, so make sure to come back here as soon as you get any available.