Petyr Baelish
Olenna conspires with Petyr Baelish to murder Joffrey, so as to protect Margaery from Joffrey's beastly nature – it is Olenna who actually does the deed, using a poison Petyr arranged to have smuggled into the wedding on Sansa's person. Quietly admitting her actions, Olenna advises Margaery to become acquainted with her new match, Tommen Baratheon, Joffrey's younger brother and heir, before Cersei turns him against her. Olenna returns to Highgarden shortly afterwards.