Luka

Luka is both the age-old mentor to Justin and Marissa and protector of the treasures that are hidden away in the lost forest of Brovania who first appeared in the forty-seventh book in the original Goosebumps book series, Legend of the Lost Legend.

Legend of the Lost Legend
A hairy large and snarling beast enters into the room that Justin and Marissa are in as Ivanna tells them they have nothing to worry about since it's name is Luka. Luka, it turns out, is going to teach the sibling's how to survive their test in the Fantasy Forest. When Justin awakes the next morning, he discovers that he's in the middle of a field. He finds Marissa, who had gotten up before him and done some exploring. Luka guides the kids down a path in the forest, and the kids find two empty backpacks, which they take. Luka begins chasing a squirrel, and the kids lose track of him. Justin finds a note from Ivanna in his pocket, and it says that the kids will be doomed if they lose Luka. Near the end of the book, he reveals that he is actually a human, and his family has helped to protect the treasures in the forests of Brovania. He reveals that Ivanna and all of the strange creatures that the kids encountered were his creation. Luka gives Justin a silver chest with the duo's reward inside. Luka is instructed to follow Silverdog back to their campsite. While following the dog, the siblings run into their father, and Justin presents the silver chest to him. When the trio opens the chest, they find a large egg inside. Silverdog reappears, and Justin asks the dog to take them back to Luka, which the dog does. Luka is surprised that the family doesn't want the Eternal Egg of Truth, but they explain that they would prefer the Lost Legend. Luka takes back the egg, and he says he can direct the trio to the people who possess the document but mentions they probably won't be willing to part with it.

Physical appearance
When in disguise, Luka takes on the form of an large, hairy, and snarling beast that is part of his disguise. When not in disguise, he is described as an aging old man that has an white thin hair, blue eyes and has on a gray robe.

Personality
Luka both has a bit of macabre element to him yet is very wise to both Marissa and Justin as he is both their mentor for trying to survive in the Forest.