Belle is a character which could so far only be seen at E3 2017 as a big figure in the Sea of Thieves Booth. Some more details can be found on the website of the people who made the figure: Spur Creative. There, she is described like this:
Belle is an ethereal, ghostly character who floats around the seas on a barnacle infested buoy with glowing blue eyes and a lit lantern held in her hand.
Appeared in: E3 2017 Sea of Thieves Booth
Location: On a buoy
Organisation: Buoy Surfers
Here are some other pictures from the Spur Creative Website: