The Bouldering Guy is a character from The Legend of Zelda: A Link Between Worlds. As his name implies, he is a boulderer--being a method of rock climbing that does not use ropes or harnesses--who at some point becomes stranded on Hyrule's Death Mountain. His Lorule counterpart is the Mysterious Man.
After becoming stranded on Death Mountain, he wrote a message that he put in a bottle for others to find, asking whomever reads it to bring him Premium Milk to restore his energy. Link eventually finds his message, which has somehow made its way to the south-eastern shores of Lake Hylia, and duly brings the message to the owner of Kakariko Village's Milk Bar, who provides him with a bottle of the Bouldering Guy's coveted Premium Milk.
Link eventually finds the Bouldering Guy in eastern Death Mountain and brings him his milk. The man immediately downs the milk, and with his energy restored is able to get off the mountain. He lets Link keep the empty bottle as thanks. After this, he may be found lounging in the Milk Bar.