Erisk is a mage scientist that is investigating a mysterious fountain in Dernel Jungle that has healing properties. He is discovered during the Grand Youth quest, when a bottle of Healing Water is required to heal Monte's grandfather. Along with his partner, Isfurus, he's developed a method of carrying the water in a specially prepared bottle, so that it doesn't lose it's healing properties.
Erisk is found in the heart of the fountain in Dernel Jungle. To reach him, you should jump into the top of the fountain, swimming through a hole in the bottom.