Fern unlocks when you collect 35 or more restore cells in a single run :)
I'm really glad you're enjoying the game, it means so much to hear that it's been a fun (and maybe slightly dangerous) distraction from work.
P.S: If you have a moment to rate the game, that would help a ton! Positive ratings really help get the game in front of more players, which makes a big difference for me. Thanks again for playing!