Maestro Forte was the great musician of the Beast's castle, entertaining the Prince with his music. He was changed into an organ during the Beast's curse. But unlike the other inhabitants of the castle, he is very much satisfied with his new form. Before the curse, his music did not attract the attention of the Prince.
Now that the Prince is a beast, he feels the need to play music to appease his soul. Forte therefore doesn't want Belle to fall in love and then become useless again in the eyes of the Prince once again He uses his apprentice Fife to spy on the castle since he is chained to a wall and cannot see beyond it.