Elizabeth Fisher
On the anniversary of his father's fake death, Chuck spots Elizabeth putting flowers on his father's grave. This leads him to go on a hunt to find her and she tells him that she is his real mother. Then Chuck's hotel, the Empire, started getting bad press and he had no choice but to sign his hotel over to his mother. It turns out that his mother was working with his evil uncle, Jack Bass, at the time and Jack just wanted Chuck's hotel. This lead to the breakup of Chuck and Blair and his whole world started to fall apart. Chuck traded Blair for his hotel back and he realized it was the worst mistake of his life. Elizabeth told Chuck that she wasn't his real mother and then left him, alone. Then in season 5 it was revealed that Elizabeth was his real mother.