In crossy road, some specific characters require a character to unlock. For example, the character "Duck" can be unlocked by playing to the maximum. Completing achievements can also help you unlock secret characters, so check the achievement list for tasks that can lead to unlocking.
Occasionally, the game also holds new events or updates, which can introduce new secret characters, so keep an eye out for official announcements. Finally, some characters can be purchased directly in the game. To increase your chances of unlocking, you should play often and try to save money to buy multiple mystery boxes at once.