Jojo's Bizarre Adventure

Fight as Dio:
Highlight "Alessy" at the character selection screen and press Start. Highlight "D'Bo" and press Start. Highlight "Chaca" and press Start. Highlight "Milder" and press Start three times. A hand and pencil will draw Dio's icon to confirm correct code entry.

Fight as Shadow Dio:
Unlock Dio, then highlight him and press Start ten times.

Fight as Young Joseph:
Highlight "Jotaro" at the character selection screen and press Start. Highlight "Polnareff" and press Start. Highlight "Joseph" and press Start. Highlight "Kakyoin" and press Start. Highlight "Iggi" and press Start. Highlight "Avdol" and press Start. Highlight "Joseph" again and Young Joseph will eventually appear.

All bonus characters:
Access the operator's menu. Select "Configuration", then "Game". On player one's side, press Up, Down, Right, Left, 1, 2, 3, 4. A sound will confirm correct code entry. Dio, Shadow Dio, and Young Joseph will be unlocked.