How and Why Should I Perform Soul Boost?

Soul boosting in the game is practically where you'd passively upgrade every single stat your character has. Along with improving on your character's basic stats, you also increase its overall battle effectivity rating. Soul boosting mainly increases the following fields:

Srike Attack - increases the effectiveness of your melee strike and its ability to penetrate the defense of the enemy.

Strike Defense - increases your effective defense against your opponent's melee attacks / combos.

Blast Attack - increases the effectiveness of your ranged attacks (Ki Blasts) and its ability to penetrate the defense of the enemy.

Blast Defense - increases your effective defense against your opponent's ranged attacks (Ki Blasts) / combos.

Health - Increases effective Hitpoints of your character

Class Up - Unlocks next level cap

Each panel would usually require a certain amount of soul crystal along with a certain amount of zeni to actually upgrade. If you happen to not have enough soul crystals to push through with an upgrade, you'll be given the option "How to Get" within the upgrade panel itself -- which would list a number of chapters or event activities which you can perform and earn the specified crystal from. You can freely re-do the given chapter / or event a number of times to earn as much crystal as you want.

Otherwise, a good way to earn soul boost crystals is by sending high level characters to do your adventures. When the adventure turns out to be a "Huge Success", it would usually reward you with a ton of soul boost crystals along with zenies.