Simple Digimon Game - Suggestions Welcome

[nilamo]

(also, all the monsters are named after and based on actual Digimon, since this is entirely non-commercial and I have no plans for distributing the game)

I love to be the fun-killer, but being non-commercial doesn't protect you from copyright infringement. Even if it was open source, and you gave it away for free, you could still be sued.

But don't take my word for it, google around and you'll see that to be stated again and again.
It's why games like mario clones don't last too long before they're taken down by isps.

There's a few exceptions that are noteworthy. For example, you can't copyright rules (but you CAN copyright the medium the rules are distributed on, so don't photocopy a rulebook and share it). So risk/monopoly clones are fine, as long as all the names/art is changed.
Furthermore, something that is common enough to be considered public knowledge have essentially expired their copyright. These things are very rare, and if you try to use that, you'll probably lose anyway. Breakout type games fit this mold, since the game is just a paddle hitting a block into other blocks. Tetris is almost at that point, since it's just rotating tetriminos, but the company that owns the Tetris branding still sues people unconditionally anyway (though they do lose now and then, if the art is different, number of columns is different, etc).

Which is a whole lot of words to say a simple thing: don't use Digimon.
You can easily have the same game, without using any of the copyrighted material, simply by calling them "summonable beasts" and making up unique names.