Story
Reuben said that he didn't know where Core was, but that you could find clues in his hideout in the west of the Basfelt Ruins. Go to Basfelt Waterfall to check out the ruins and find Reuben's Hideout.
After completion (spoilers)
"Reuben's Hideout is located in the west of the Basfelt Ruins." Reuben said that he didn't know where Core was, but that you could find clues in his hideout in the west of the Basfelt Ruins. You gained information from graverobbers at the Basfelt Waterfall and found Reuben's Hideout.