Great start so far. Definitely looking forward to more. Just a couple notes.
For starters, I don't know why the game can't be played entirely with the arrow keys/enter. While this is MOSTLY true for the game, selecting a rival to challenge must be done with the mouse. I'm not sure if that's an oversight or what.
Not really a bug, but something to note. After a certain height, the player and rivals can grow faster than can be drained. While this isn't strictly true for the player (because they can sit and allow themselves to be drained), it probably warrants a hard cap on the growth of rivals. I was actually gambling constantly with Blue, and because we both grew more than 100 every 5 games, it was impossible for either of us to outpace the growth. For Blue, this was at about 800 height (when the growth triggers).
Another thing to note is that, for White, she said surviving for 3 minutes would count as a win. The timer actually means nothing, neither win or lose. I'd be fine if it counted as a loss, but it would need to extend into the shed if it does.