Finish stage 8-4. This stage is unlockable during your second play. Unlocking Sharaku will also remove the exclamation marks on Ham Egg and Don Dracula.
At the "THE END" screen after the staff roll, do not press start. Wait until an event occurs. This will unlock Sharaku in the character list.
Beating the game in any mode (easy, normal, or hard) Unlocks the stage select option from the pause menu. Astro Boy can still build up his stats and even meet new characters.
Infinite EX Points
Full EX Gauge
Always Low Time