Duke Fleed (デューク・フリード Dyūku Furīdo?) is the Crown Prince and one of the few survivors of Planet Fleed, which was destroyed by the forces of King Vega. He has found a new home on planet Earth and vowed to protect his new homeland and take revenge on King Vega and his minions. With the robot Grendizer at his command, Duke Fleed fights back invasion after invasion in the 74 episodes of the UFO Robo Grendizer series, with the help of former Mazinger Z pilot Koji Kabuto, who serves as his sidekick.

Duke Fleed's Earth name is Daisuke Umon (宇門大介 Umon Daisuke?). He purports to be a normal human and his cover is that of the son of Professor Umon, head of the Space Science Lab. He works as an employee of farmer Danbei Makiba. At first Makiba, his teenage daughter Hikaru and young son Goro know nothing of Daisuke's secret identity and nature as an alien prince. Makiba, a hot-tempered source of comic relief, just bullies Daisuke to get on with his work (which he frequently neglects in order to go and fight Vegans) and sees him as not good enough for his daughter who is enamoured with the young man. However, various incidents lead them to discover who he really is.

Later on, it is revealed that Duke’s younger sister Maria Grace Fleed has also survived the planet's destruction and fled to Earth. She joins Duke's team in its fight against Vega.

In the show's last episodes, it is shown that Duke Fleed was engaged to King Vega’s daughter, Princess Rubina, prior to the attack on Fleed.

After King Vega's final defeat, Duke and Maria Grace return to Fleed to help rebuild their homeland.

In continental Europe and in the province of Quebec he is named Actarus.