Brother of Molly (Prewett) Weasley (JKR, OP9), who died heroically in the first wizarding war.
Fabian and his brother Gideon were among the "best witches and wizards of the age" who were killed during theVoldemort years (PS4). They were part of the original Order of the Phoenix. It took five Death Eaters, including Antonin Dolohov, to kill Fabian and Gideon (OP25). Moody said they died like heroes (OP9).
Molly Weasley gave Harry Potter Fabian's watch on his seventeenth birthday. Fabian was not too carefull with his possessions, so it was battered (DH7).
Part of the Prewett family, brother to Gideon and Molly Weasley-Prewett.
Two of the best wizards of their age, according to Mad-eye Moody (OP9).
It may or may not be a coincidence that Fred and George share the same initials as Fabian and Gideon.