Press the arrow keys to move. So far, the only thing you can do is press the mercy button, since I'm still working on the others. If you want to play through the whole game so far, I'd recommend naming you/the fallen human "Frisk" or "Chara" (no quotation marks.) Don't ask, but do it if you just want to see what I've done so far.
This is a work in progress, made a year ago. I looked at it today and realized I could continue, so I added a few key components: naming the human, your level (it'll go up as you defeat more monsters, if I decide to add more), and your HP.