The ghost of Syd Barrett, WISH YOU WERE HERE from PINK FLOYD

When Nick Mason, the Pink Floyd's drummer gives in 1987 an interview to a journalist to talk about the news of the band, and he mentions randomly the name of Syd Barrett, the journalist asks to him at a certain point "Wait a minute, who is Syd Barrett ?"
Even if the time he spent in one of the major band in rock history is minor, the impact he had on the others members of the band and on their following compositions is huge, and not only in term of musical style.
So, who is this man at the beginning of one of the most melodramatic episode in rock history ? And before, what legacy Barrett does it leave behind him after having left the band in 1968 ?
Who is Syd Barrett ?