Personality particularly, all of my friend's/players PCs I've found memorable are the ones with clear and specific personality traits, and are descriptive about their body language and behaviours. Mauve the wine-aunt plant fairy or Squidunk the toad loving gnome were infinitely more memorable than the Sorlock who thought he was mysterious and his girlfriend who entire character was "sorlock's love interest". I legit can't remember their names and that campaign ended two months ago, compared to about 5 years for Squidunk and 8 or 9 for Mauve.