Definitely agree that both "on & on" and "who am I" are easier than "Link", and thus might make more sense as an introductory level.
Some other notes, after trying the first level of each planet for a few minutes, and finishing some of the easier ones:
Having to press escape on every loss to see the mouse cursor slightly interrupts, and thus slows down, the action of trying again to learn from my mistakes. It is more disruptive with the webgl build (at least in Firefox) since it also quit out of fullscreen mode at the same time. If the cursor is hidden on purpose during the level, could it be un-hidden automatically on a loss, to speed that up?
Like Interrealm Games, I was surprised by the collision behavior. In particular:
1) When I tried to "hug" side walls, I'd slide sideways through them and off the path, even though they looked to be solid. If that's intended, that's fine, but some sort of visual indication of the solidity of the geometry would make it less surprising.
2) When I was really short on a jump, I could pass into seemingly solid blocks (especially on "Link" where the blocks of the path are very tall) and seemed to keep going, instead of triggering the loss right away. This also slows down the action of retrying to learn from mistakes.
3) In "calling for you", the rotating paths would change from something I could travel on, to something I fell through, as they rotated. If that's part of the level's intended difficulty, that's fine, but it would be more clear if the player slid along the angled surface of the path rather than dropping straight down through something that appears otherwise solid, and had been solid until that moment.
I didn't notice the button to choose other levels after selecting a planet at first. Maybe larger or higher contrast navigation arrows would stand out more.
The levels that are primarily a single color (light gray, white, or near-black) in color don't always have a lot of contrast within them (except for the pink lethal objects). Unless contrast and/or color perception are part of the intended difficulty, maybe tweaking the colors a bit, or else adding a "high contrast mode" setting would be helpful.
Once a world is chosen, I didn't see an option to go back to the world selection screen, only to choose a level for that world. Maybe I just missed it, but it's nicer to have a back button there than to have to start a level and immediately fail it on purpose to go back.