Even though you can't choose Bunny as your starting character, you can still unlock her early in the game. Here's everything you need to know to unlock Bunny in The First Descendant

You can unlock Bunny in The First Descendant by completing her respective subquests. After reaching the second battlefield in Kingston, you will receive a subquest called Obtaining Bunny Suit Basic Materials, which has multiple stages.

Complete the main story and you should passively unlock the Bunny research materials, as both require you to complete the same missions. Bunny is a sort of Tutorial Descendant, meant to be the very first one you unlock, similar to the Rhyno in Warframe.

Once you have all the research materials, go to Anais (in the western part of Albion in a round research room) and start researching. Wait a minute, then all you have to do is zoom around in your new bunny suit.