Dewdrop is a Mercury Djinni in Golden Sun: Dark Dawn.
Dewdrop is found at the Teppe Ruins, north of Te Rya Village. However, the Trusty Staff must be purchased from the artifacts section from the female shopkeeper in Te Rya Village. After obtaining such staff, go to the Teppe Ruins and hit Dewdrop with a fireball, causing it to go into the river. Next, go into the hut and give the northernmost woman the staff. She will go fishing with it, and will reel him in. Finally, all you need to do is to claim your prize.