You will be able to find Crusty Sean in his food truck which is located in the middle of Inkopolois Square. Crusty Sean offers food booster items that will provide you with bonuses such as special abilities, money earning bonuses, or experience bonuses which can then be used in online matches .
These food booster items can be purchased using Meal Tickets which you can get through the single-player mode. Your main source of funds for these food booster items is going to be through completing games of the multiplayer Salmon Run mode which will enable you to unlock Bonus Orbs that will give you various rewards depending on the rarity of the orb at Crusty Sean's food truck.
Listed below are the food items Crusty Sean offers and their bonuses.
Double-Fried Super Shwaffle:
Battle Cash doubled for 20 matches.
Super Seanwich:
Battle experience doubled for 20 matches.
Deep Fried Shwaffle:
Battle Cash increased by 50% for 20 matches.
Crusty Seanwich:
Battle experience increased by 50% for 20 matches.