How can I keep track of the subcategories? Everytime i open the game's splits, the subcategory is set by default on Mario, changing the World Record.

If I understand correctly you want it to show the WR for say a Luigi run? Assuming that, here's what you need to do.

On your splits, go to the "Additional Info" tab and set your character.

Edit your layout and edit your "World Record" section. There's a section that says "Filter Leaderboard By" and put a checkmark in the "Variables" section.

Hope that answers your question.