BF Mode is the visual novel portion of the game.
Essentially, you can pick whichever kare you want to romance.
Genki will not be used up.
Because it's a fairly new gameplay to the game, there are only a few kares to choose from, but more is being added.
Once you choose your kare, click the pink button and you will see your current progress with him. Click the pink button again to start reading.
Throughout the story, you will be prompted to choose a certain choice. What you choose affects what ending you get: either a friend end or a love end.
Your choices will affect the tokimeki meter (ときめきメーター) on your kare's progress page. Getting past the arrows and getting a pink bar before the first end means a love end; anything below that arrow at the first end means a friend end.
Occasionally, you will also face bosses at the end of each chapter in the form of other kares trying to confess or distract you from your main kare. Your honmei kare will be used to attack the boss.
There are walkthroughs on the Japanese wiki. The structured table are the choices to raise the tokimeki meter for the love end. The paragraph at the very bottom of the table are the choices for the friend end.
Unlocking each ending will raise your team's attack points.