Might be easier to run a limited palette/pattern dither in Photoshop set to the DS' palette limitations and
then implement the portraits. Letting the DS data do it natively is much more likely to cause palette and shaping shenanigans.
(for reference, DS can support up to 256 colors on any one palette, and can have up to 16 unique palettes rendering onscreen at once)
For instance:
Your Genya has a lot of shade blotching from the color limiting (which I'm assuming is done natively by the DS emulator, not sure what you've done to the images prior to insertion).
Compared to this Genya, restricted to only 64 colors with a uniform pattern dither. Those blotches are all but nonexistent now and everything looks a bit cleaner.