Chapter I introduces the characters and some key game terms and controls. Here's a primer!
Gacha - Final Fantasy Brave Exvius is what's known as a gacha game, in which you can "pull" resources (randomly!) using in-game currency that can be acquired for free or for cash purchase. I was not employed when I played FFBE, so was strictly a free player--and yet, did pretty damned well with the game, if I do say so myself!
Lapis - The in-game currency used for pulling resources. Lapis can be acquired from doing story missions, beating trials, or completing limited-time events.
Visions - The term FFBE uses for their "pulled" resources, visions are characters, either from other Final Fantasy games, from cross-over events, or unique to FFBE. You use your Visions to form a party that fight for you in battles.
Rarity - I don't think this comes up in the game script, but I'll explain anyway. Each vision has a different rarity. 1* and 2* are the most basic of visions; they're really not worth using, although when the game launched, some of them had decent TMRs. 1* and 2* units are pulled with friend points, a currency you get for logging in each day and sending "gifts" to your friends. 3* are the base rarity and most common acquired via pulling; when you get one, a blue crystal comes down the screen before breaking open to reveal your vision obtained. 4* are one step up and more rare than 3*; they break from a golden yellow crystal. Early on in the game, the highest rarity and least common were 5* units that broke from rainbow crystals.
Trust Master Rewards (TMRs) - As you use a vision in battle, it acquires Trust Master points. When the points reach 100%, you obtain a reward. This reward can be an excellent weapon or equipment, or it can be an ability like Dual Wield or Doublehand.
The Farplane - sort of another dimension. Short-term events and difficult trials can be found there.
The Veritas - Mysterious people dressed in colored armour. We don't know much about them as the story starts.
Grandshelt - the Kingdom Rain and Lasswell (our heroes) are from. They are knights of Grandshelt.
Rain - Son of Raegan, Rain followed in his father's footsteps and became a knight. He does not bear much love for his father, however. Rain is a bit carefree at times, but is quick to help others out and has a big heart.
Lasswell - Rain's childhood friend. Lasswell became an orphan at a young age, and was taken in by Raegan so was raised along with Rain.
Raegen - The finest knight of Grandshelt, although he's been missing for a number of years.
Fina - A mysterious girl that first appeared to Rain and Lasswell as a being encased in a crystal. She has forgotten most of her memories and is quite naive as the game starts.