Marwyn is lanky and has red hair.
A Clash of Kings
A Feast for Crows
Marillion, a favorite singer of Lady Lysa Arryn, mocks Marwyn as "Ser Ding-Dong" during his tenure at the Eyrie. For that impertinence, Marwyn vows to have the singer's tongue removed but Lysa forbids it.
Upon the arrival of Lord Petyr Baelish, Marwyn is removed from his position and he is replaced by one of Baelish's men, Ser Lothor Brune. Marwyn later returns to the Eyrie to hear Marillion confess to the murder of Lady Lysa.