In Crossy Road, you can unlock secret characters in a variety of ways. One of the popular methods is using coins to buy mystery boxes, which can contain secret characters. Each mystery box will have a chance to open a random character, including secret characters.
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.