Harry's maternal aunt. She's an unpleasant woman and doesn't like Harry, but she and Vernon agree to take Harry in because her sister and Harry's mother, Lily, sacrificed herself for Harry, which gives Harry protection if he lives with Petunia. Harry later learns that Petunia wanted to attend Hogwarts as a child.
Aunt Petunia Character Timeline in Harry Potter and the Deathly Hallows
The timeline below shows where the character Aunt Petunia appears in Harry Potter and the Deathly Hallows. The colored dots and icons indicate which themes are associated with that appearance.
...He takes his time putting down the mirror fragment and goes downstairs to find Vernon, Petunia, and Dudley all dressed for traveling. Vernon tells Harry he's decided he doesn't believe it... (full context)