How he is one of the most moral characters on the show (or tries to be), especially considering he is a pirate; he is loyal, wants freedom for himself and his ‘brothers’, is able to put things behind him for the greater good, realistic… (which makes his character in TI all the more tragic)
His on-going character development (even if it will likely break my heart as he goes from innocent pirate puppy to alcoholic, rude and scary scoundrel)
His loyalty to both the crew and Flint(’s plan), and how he tries to handle it when these loyalties conflict
How he is boatswain, then quartermaster, then first mate, and handles each position with equal skill
The fact that this guy has survived impressment and being tortured by the Navy and is still determined not to give up and instead stands against England and civilisation
The fact that he has a lot of hidden depth, whether it’s the initial question of whether he will support Flint or not in S2, the reveal of his backstory in the same season, to him taking on a more active role in S3 and using said backstory to his advantage
“You haven’t gotten yourself laid yet.”/“[Let us] forego, just this once, a fuck tent.”
Snarky Billy
Bad-Ass Billy
Fighter Billy
…and how he is still somehow a cinnamon roll
His random cussing and use of swear words
His accessories (his rainbow belt in S1 and his necklace in S2 and onward)