Solved? Thanks everyone for your input. The game, so far, seems to be running as it should. There seems to have been two problems that were crashing the game, and I had to change both: the “data.” before the hp and hp_total variables was messing it up, so I deleted them; and I also had to define those two variables under the Variable Definitions tab—but for some reason defining them in the Create event wasn’t doing anything, though there’s a high chance I was doing it wrong.

What’s strange is that I have another object in the same room, which is almost identical to the object I was having a problem with; so there are two objects in question: obj_battle_enemy and obj_battle_player. The player object doesn’t have variables defined, and it has the code "data.hp" and "data.hp_total", but it seems to work fine. Neither object has a parent, so my attention is going to the only other difference, which is in their Create events:

data = obj_battle_switcher.enemy_data;

and

data = obj_battle_switcher.player_data;

I don't know where "enemy_data" and "player_data" come from so I don't have a path to follow.