Instructions
Load your copy of "Mario Kart Wii" to the main menu. Select the "Time Trial" option. This allows you to race against ghost characters that can't attack you so you can get the fastest possible times in each track.
Select the character you use best. The object of Time Trial is to get the best possible times, so pick a character with which you feel comfortable.
Beat the first ghost character by seven seconds or more on all 24 tracks. This unlocks a staff ghost -- these are characters that were used by the game's development team staff members. When you unlock the 24th staff ghost, you will unlock the Sprinter.