On PC the game saves the game in a local folder. I don’t think it’s account-based, but based on local installation. So if you both play on the same physical machine, you share the Saved folder (ie. the latter player overwrites the previous save files).
However, on PC there is a workaround. You can manually copy your save files to another folder, let the other person play, then copy their saved game files to another folder and copy yours back into the original folder. This way you can swap between accounts. It’s a bit of a hassle, though.
Another workaround is the much praised Mikey’s Toolbox which lets you have multiple characters in a single save file.
As for recovering your old character: depending on how long the other person had played, one of the backup save files in the Saved folder may still contain your play data. The game-db file is the most recent and should contain the other person’s save data. Copy this somewhere else to safety and rename one of the backup files to game.db and see whether your character loads. (Sorry, I’m not at my home computer right now so I can’t tell you the exact folder paths or file names.)