Link first meets Rosso in his house. Rosso is seen throwing rocks away after a recent volcanic explosion caused several rocks and boulder to crash near his house. He offers Link the Power Glove, allowing him to lift small rocks as well, while also joking that it would be nice if Link could help him. If the young hero actually does destroy all the rocks around Rosso's house, he will be given 50 Rupees as thanks.
Later in the game, Yuga discovers Rosso is one of the Seven Sages, due to his lineage as a descendant of the Seven Wise Men, and turns him into a Portrait. His painting is later used, alongside the rest of the sages' as well as Princess Zelda's to revive Ganon. The paintings of the Seven Sages are then spread throughout Lorule, with Rosso's being taken to the Ice Ruins. Link ventures through the dungeon and saves Rosso, restoring him to normal. Grateful, Rosso thanks Link and wishes him good luck on the rest of his quest. After the last sage is freed, Rosso unites his power with the rest of the sages' to grant Link the Triforce of Courage.
As part of the A Link Between Worlds DLC, Rosso's painting Portrait can be seen in Yuga's Victory Cutscene along with the portraits of the other Seven Sages and Zelda.
Descendant of Darunia
Due to his appearance, the Goron symbol on his belt, being the Sage of Fire, and a descendent of the sages, some fans theorize that he is a descendent of Darunia. This would mean that Gorons are capable of bearing children with Hylians.