As a brand ambassador, promotional model, or event staff member how you are dressed and how you present yourself is important but.. this is absolutely NOT everything.
Your personality and your professionalism are most important.
There’s a big chance you will be interacting with guests for many hours while working and you will maybe need to look like you are enjoying yourself even when you are not. You need to relate to consumers, get along with the client and other staff at the event. It’s a good idea to also have a deep local knowledge or technical understanding about the product and services you are endorsing.
Don’t forget to smile, be polite and be professional at all times.