Check out Sebille's companion quest description:
Stage 1: Fort Joy
The quest starts right after you recruit Sebille to your party.
Visit Stingtail who sleeps near the entrance to Caverns.
He is killed by Sebille after she gets information so if you are accompanied by The Red Prince then you should allow him to speak first. Then, let Sebille speak with Griff. He suggests her to talk to Zaleksar.
Warning: If you told Griff who stole his oranges then he will send people to kill Stingtail. If they manage to kill him before Sebille does, you will lose a chance to continue this quest.
Zaleksar can be found in Hollow Marshes. Use Persuasion to make him talk. Otherwise, he will fight you.
Stage 2: Reaper's Coast
Reach Reaper's Coast and find Roost. There are two sources from which you can learn about his whereabouts. If you have Ifan in your party then you can speak with Baran in The Black Bull. Otherwise, you must head to the elven camp and convince them to let you participate in a ritual. If Saheila survived the events on Fort Joy then the elves will ask you to rescue her from Roost.
Go to Sawmill and confront Roost. He has information about Sebille's master. Regardless of how the conversation goes, you will have to fight with Roost.
Stage 3: The Nameless Isle
Sebille's master is called Shadow Prince and he can be found in Lava Fields.
Go there and let Sebille talk with him. Shadow Prince will try to turn her against you. You must follow the instructions that she gave you before the conversation. After that you will have to fight with Shadow Prince.
If you head to Heart Of Mother Tree after Shadow Prince's death then Sebille will be asked to become a new Mother Tree. If you agree Sebille will receive +3 to Memory. If not then you will become enemy to everyone in Temple Of Tir-Cendelius.