Antonyms for the phrase "most up front" include obscure, hidden, secretive, discreet, unobtrusive, and reserved. An obscure person may hide their intentions or actions, while someone who is hidden may not be easily visible or accessible. Someone who is secretive will deliberately conceal information, while a discreet person is careful not to draw attention to themselves. An unobtrusive person is inconspicuous or not easily noticeable, while a reserved person is generally introverted and keeps to themselves. These antonyms contrast with "most up front," which connotes a frank, open, and transparent approach.