Luka(Rusalka in Japanese) is a major supporting character from Secret of Mana. She is the 200-year-old Rusalka(an Undine Ghost in Slavic Folklore) Sage of Water, but looks no older than 18. She is friends with Jema and Undine.
Luka lives in the Water Palace and guards the Mana Seed of Water. The flow of water brings her news from all around the world. She helps Randi to understand his mission and reveals to him the dangers that the Empire is trying to bring upon the world. She also introduces the group to the elemental spirits.
After Randi, Primm and Popoi save Undine and return the stolen Mana Seed of Water to the Water Temple, they find Luka held hostage by Geshtar, and have no other choice than letting the villain break the seal on the Mana Seed, before fighting Jaberwocky.
From then on, Luka keeps providing the party with guidance and advice.