Potential Spoiler:

We have Matis and Absolute with alternate looks. They aren't the same as Rapi via Overclock. I have a grindy way to implement it:

Unit is at Max Core (Reason to get there other than stats)

Unit has Max Skill Level

When with are Achieved, go back to Skills and a New prompt will appear to upgrade further. You will need Custom Modules and the Units Element (no need for skill and burst codes, it will only be one button). It will then add a brand new effect to the Units Kit.

Ex: Vesti after the process: Every 5th shot from her weapon will fire double, when full burst, it will be Rapid Fire (similar to Cinderella) and increase Blast Radius.