Nate I tried Unity like Misaki said, but I couldn't figure out how to run it.
Then I tried the Java runtime with IntelliJ IDEA, and it worked with the Spineboy example (but it gave the error "finished with non-zero exit value -1073740791", despite the code saying "System.exit(0)").
For anyone who might want to do the same and stumbled on this post, this is what I needed to do so that I could do the same with the characters:
-Create a folder in the assets folder next to Spineboy that contains the atlas, json, and PNGs of said character.
-Remove the line "json.setScale(0.66f);" (or maybe change 0.66f to 1).
-Change the code so it gives the atlas and json of your character.
-Change the width and height of the frame buffer in the code. You can make it the same as the width and height in the json, and then increase them if they're not enough.
-Change the x and y in setPosition. You can also use the x and y from the json's skeleton (but positive).
-Change the animation from "run" to whatever the name is of your character's animation.
That was everything I needed to do.
You may possibly need to change renderer.setPremultipliedAlpha(true) to false instead of true, in case the atlas says so. And also you could change the fps to match the skeleton in the json.
Thank you both for your help!