For the character from A Song of Ice and Fire / Game of Thrones. Wylla Manderly is the granddaughter of Lord Wyman Manderly, lord of White Harbor which is the North's only major city and sea port.
House Manderly was originally from the Reach but was exiled from there a thousand years ago- but the King of the North gave them shelter and the rose up to become one of the major houses in the North and one of House Stark's most loyal supporters. They are one of the very few noble families in the North that follow the Seven instead of the Old Gods. Their sigil is a Merman holding a trident and their castle is called Merman's Court, and is lavishly decorated with murals and paintings of mermaids, sea creatures and underwater scenes.
Wylla is 15 in A Dance of Dragons, with long blond hair she dies a bright green (a Tyroshi fashion). She sits beside her grandfather in the Merman's Court and speaks her opinion fearlessly, even under threats from Lord Wyman.