The Wedding Bouquet is a promotional item from Fire Emblem: Awakening, and is one of the rewards for completing the DLC chapter Smash Brethren 2. Any female character can access the Bride class by using the Wedding Bouquet when she is Level 10 or over (or any Level, if she's in a promoted class).
Using the Wedding Bouquet does not permanently unlock the Bride class for a unit. If reclassed out of the Bride class, another Wedding Bouquet is required to reclass into it again. Smash Brethren 2 will reward the player the Wedding Bouquet each time it is completed.