I agree with others that the platformer control is quite awkward. Also the Undertale part I was moving my heart to the corner to minimize the damage because it is quite impossible for me to dodge the obstacles (the damage was higher if I try to dodge). You may want to adjust the colliders to make them less punishing.
Also a bug: My heart is invisible (only visible for 1s when I take damage) lol, it's like in hardcore mode
Overall quite nice, it would be better to not just implement existing games' mechanics but try to fuse them together to form new levels.