Well done - you could have totally moved away from Celeste and stick more to the Portal look, but good job anyway.
Couple of comments:
- ESDF + mouse works well - slightly awkward for jumping precisely
- as other pointed, the portal physic appears to be broken when you move into a portal (e.g the player will instantly go "back"). 2d makes it impossible (unless you flip the screen when getting out of a portal to keep momentum and keyboard aligned!!)
- music is a bit "depressing" (?) RubyRed I know you can do better :]