The cosplayer's hair is short and dark black, mimicking Howl's natural hair shade rather than his dyed light blond hair in the beginning half of the movie.

Howl has black hair, so it seems like the poster expects you to have seen the movie already. In the background, we can see supporting characters like Markl and Turnip-Head.

The black gateway is a portal into Howl's psyche and unconsciousness, ... Namely, they look nearly identical to a younger Howl but with different hair. This implies more than a few things, ...