Ashi is wearing Aku or something that is Aku like.
I will now make my case.
In episode 2 we see Aku waking up in a bed decorated with flames before he removes his flaming eyebrows from a case next to his bed. Clearly fire is part of his “nature”. Ashi is pushed into some kind of smoldering ash pit and comes away coated in her black suit.
To remove the black suit, she submerges herself in a pool of running water an environment adverse to fire.
When she has a flash back to when the suit was applied to her she was a small young child and it was clearly animated to show us that it was traumatic. If she had to have it reapplied periodically she wouldn’t have flashed back to childhood but rather to simply the most recent memory of the ash pit.
Further if that is the case how did the suit grow with her and not show marks and gaps as she grew into adult hood? Indeed, given the number of injuries her rigorous training likely involved why are there no tears or rips? But if it is Aku substance she is wearing, given the nature of his shape shifting ability, the suit could grow with her and reform around damaged areas.
Note Ashi’s feet they do not look bare. This is not caused by a failure to successfully render toes as Jack’s bare feet clearly have toes. Indeed her feet aren’t even shaped like human feet, rather she has a pointed shoe shape. Clearly, she is not just covered in some kind of paint but something more substantial like that of Aku.