Choose Your Legends is a pre-release event that runs from 19th January 2017 to 5:59 AM (UTC), 1st February 2017, right before the official launch of the game.
During this time period, users can vote for their favourite character(s) once a day from the Choose Your Legends mini-website.
The two highest-scoring male and female characters will appear in Fire Emblem Heroes in special Choose Your Legends costumes. These characters can be summoned using earned or purchased Orbs.
In addition, from March until December, Nintendo will distribute monthly calendar wallpapers featuring the top 10 scoring male and female characters via their Twitter account. (The January and February calendars feature Alphonse, Sharena and Veronica.)
Users can vote from a humongous pool of 793 playable and non-playable characters from throughout the entire series. There are two ways to vote: By choosing a game title or randomly.
Bonus: If you link your Nintendo account, you will receive 200 Platinum Coins once the event is over. Linking an account also helps you keep track of your votes via the Battle Ballot.