Nice Guy is an informal term for an adult male who portrays himself with characteristics such as being gentle, compassionate, sensitive and vulnerable. The term is used both positively and negatively.
Nice Guy fashion is generally non-descript (similar to the Geek or Animecore aesthetics, if anything), but are noted for one fashion item; the fedora (technically a trilby hat, but for the sake of simplicity, we'll just call it the fedora). They might also wear trenchcoats (in an attempt to look mysterious) or suits (in an attempt to appear suave and sophisticated), but they are guaranteed to wear a fedora of some form. Their casual wear
The Yin and The Yang
With Nice Guys, as stated above, they portray themselves as being gentle, compassionate, sensitive, and vulnerable. The Yin, however, is the stereotype everybody is familiar with; that this demeanor of theirs is a front and an attempt to manipulate girls into having sex or dating them because they feel entitled to it. These are the type of people who have a tendency to call women "sluts", "whores", "bitches", and all sorts of derogatory words when they politely turn them down for whatever reason (be it they have a boyfriend/husband/fiance that they are perfectly happy with, they just aren't interested in them, they just want to be friends, the girl they're hitting on just happens to be lesbian, or they see through the act and realize what it is they're really trying to do) and will, like clockwork, refer to themselves as "a nice guy" in all of this. After enough rejections, these individuals will typically evolve into an Incel.
The Yang of the Nice Guy, meanwhile, doesn't typically refer to themselves as a Nice Guy and do it generally because it's just the right thing to do and don't expect anything in exchange. They are there for girls not because they hope to get in their pants, but because they want to be their for their friends and offer them a shoulder because, well, that's their friend. Fortunately, in the "Nice Guy" discourse, these individuals don't get dragged into the discussion and generally just live their lives peacefully.