Who could it be

I'm sure Spongebob doesn't mind not being paid for over time.

Squidward is the other fry cook. But it isn't that busy on those days due to he is a shitty fry cook. Plus Mr. Crabs has been seen to cook.

I always thought there were more restaurants in bikini bottom that were just unmentioned because they were irrelevent to the plot. Like remember that spongebob bought Patrick a cake for his birthday? Remember the Kelp Shake?

Chum bucket. The wiener place. The pizza place. The noodle place. The taco place. All restaurants spongebob has worked at. They even fought over him. The fancy restaurant that is in a sunken ship. Just to name a few

Assuming the Kristy Krab is only open 6 days a week, closed on Sundays, it is possible that Spongebob works 6 hour shifts Monday-Thursday and seven hour shifts Friday/Saturday because they're open an hour later on weekends. That way if something goes wrong and he needs to stay late to clean or take care of an unexpected rush, Mr. Krabs has a two hour leeway where he still doesn't ha e to pay over time. I know this from experience bc I worked at an Applebees and they did it to me and it made me mad bc I was working 6 days a week and not getti g full time.

There's actually a blue fish that works as a fry cook there he's only metioned rarely and was seen like 2 or 3 times the whole time the show has been airing.

He's a keeper haha