How? It looks like a Pokemon game.
it's one of those "top-down" games, like pokemon and other nintendo games (and tons of more obviously), but saying it's a "pokemon rip-off" tells me you haven't done enough research and just looked at a picture of undertale gameplay and just assume it's a pokemon rip-off.
for example undertale doesn't let you capture monsters using a pokeball, and give you upgrades to take care of your pokemon or some stuff
if you would say it's a rip-off from earthbound i would understand that, but saying it's a rip-off to pokemon is just the funniest stuff i ever seen really
