I have already tried to port this natively, there is just not enough RAM to run it...
(BTW Undertale needs OpenGL, which is only compatible with the stock Raspbian image, not the RetroPie image)
You can see my failed attempt here: https://retropie.org.uk/forum/topic/2704/undertale-for-retropie-raspberry-pi
Sadly, Toby Fox did not release the source code in any way so I can not make the game lighter on the resources and make shaders compatible with OpenGLES. :(
