Lady Shella Whent is an old woman. She's a loyal vassal of Hoster Tully, the Lord of Riverrun. Her husband's younger brother was , Ser Oswell Whent, a member of King Aerys II Targaryen's Kingsguard.
To take Tyrion Lannister prisoner at the Inn at the Crossroads, Catelyn Stark asserts her origin as a daughter of House Tully and then tells a man of House Whent, Ser Willis Wode, that she recognizes the black bat of Harrenhal on his coat. He affirms Lady Whent's loyalty to Lord Hoster Tully or Riverrun when asked and then helps her to arrest Tyrion.
In the books
In the A Song of Ice and Fire novels, Shella Whent was the Lady of Harrenhal until she surrendered the castle to forces belonging to House Lannister during the War of the Five Kings because she lacked the means to defend it. She is considered a friend of the Night's Watch. In the fourth novel, Petyr Baelish states that she has died (without specifying the manner of her death), but it is not confirmed.