Currently I'm using a Gundam Marker Eraser, but it's so damn slow on hard parts and leaves a stain of what was painted on soft parts.
The plastic's shinyness must be maintained, so no sanding or other abrasive ways.

Try disassembling the parts as much as possible and leaving the bits to soak in rubbing alcohol (NOT acetone. acetone will eat the plastic. ...Actually, the soft parts might be okay in the acetone, I've seen it recommended for stripping paint off vinyl). Rub them with a cloth, cotton ball, q-tip, or whatever after they've been soaking for a bit

On the soft parts, I'd just try painting over it...

Thanks.
And I'm not painting anything, just removing Grafias' bodypaint
I'll have to dye a MMS3rd flesh ver5 when I get ahold of one to match her skin color... I'll have to look into that too.

Aha. Well, rubbing alchohol *should* take care of the bits on her torso and thighs. ...and I don't remember what parts of hers are soft. I thought her chests were the same hard plastic as the rest...

I always use Windsor & Newton's Brush Cleaner. It strips paint very well without risking harm to the plastic, though sometimes might require soaking for a while.

Soaked the parts in alcohol and finished removing the paint with the Gundam Marker Eraser.
There are some faint traces left... And the parts have a weird matte white coat, which seems to be easily fixable, but I'm not sure if I'll be able to restore the shinyness.
Any tips about the faint traces, shinyness, and to dye plastic?

dunno about dyeing, but try buffing the bits with some sort of soft cloth? not sure. *shrug*

Got rid of the whitening and most of the shinyness back by erasing it with an eraser XD

I guess I'll look in doll forums how to dye plastic, since doll customizers don't paint, they dye.