20th Century Boy is an enemy found in the Mystery Forest. He resembles a skinny man wearing a blue suit and pointed shoes strumming an electric guitar. He has long black hair. He is the corrupted form of Dag, Leg Horse's brother.
When Leg Horse finds his castle in the Mystery Forest, the 20th Century Boy appears and insults him, which prompts Leg Horse to attack him. After defeat, he reveals himself as a corrupted Dag who lost his mind, protecting the house waiting for his brother's return. Dag tells Leg Horse to avenge him before dying.
- "20TH CENTURY BOY blasts a powerful wave of distortion at PHILLIP! PHILLIP is blown away!" [Removes 50HP from Phillip]
- "PHILLIP starts to beatbox! 20TH CENTURY BOY becomes DISTRACTED!" [Inflicts Distract on 20th Century Boy]
- "PHILLIP inexplicably becomes SPEEDY! 20th CENTURY BOY strikes a pose!" [Inflicts Speedy on Phillip]
- "PHILLIP clenches his body and becomes MUSCLED! 20TH CENTURY BOY pouts!" [Inflicts Muscled on Phillip]
- "PHILLIP begins to shake! PHILLIP releases an unearthly groan and falls to the ground! PHILLIP suddenly stares up at 20TH CENTURY BOY! PHILLIP's eyes are glowing with a mysterious inner fire! PHILLIP silently mouths the Japanese word for 'finality'! PHILLIP releases CRUSHING WIND! It's super effective!" [Removes 9999HP from 20th Century Boy]
- "PHILLIP suddenly got 100MP! What was all that about?" [Entire Party Recovers 100MP]
- "PHILLIP suddenly got a BLOOD RAGE!" [Inflicts Blood Rage on Phillip]
- "PHILLIP is bathed in a mysterious light which grants a full restore!" [Entire Party Recovers All HP]
- 20th Century Boy is an homage to rock singer Marc Bolan. His name derives from Bolan's eponymous song 20th Century Boy . The guitar riff that plays when he appears is the opening riff of said song. His sprite in the game's files is labelled "bolan.png."