If we’re talking about only pre-ASOIAF characters (i.e, not Robert Baratheon, Ned Stark, Hoster Tully, Tywin Lannister, or Jon Arryn although he’s something of a corner case)…
Steffon Baratheon we learn a lot more about in WOIAF, as is the case with Aerys II Targaryen. Rickard and Brandon Stark we learn more about in ADWD and somewhat more in WOIAF, Rhaegar Targaryen we learn a lot more about in WOIAF and somewhat more in ACOK, etc.