Essentially, Bond refers to how close each party member is to Kasuga. That means that Kasuga himself does not have a Bond level. Everyone else does though, and you'll want to raise them all.
You can check everyone's Bond level (and how close they are to the next one) by going to Party and Check Stats
Why does Bond matter
Bond changes the rate at which party members gain XP while they are inactive. For the first half of the game, this isn't too big a factor, since you will mostly have a three or four person party. Once you get your fifth and sixth party members though, you'll have to choose which three will go into battle alongside Kasuga. As the main character, he must fight in every battle.
Initially, these characters not in battle will gain XP at 0.5 the rate of those in battle. Raising their Bond raises this rate by 0.1 each time.
How to raise Bond
The easiest way is in battle. For every battle they are present for, a party member will gain +5 Bond level.
You can also gain Bond by attending a Cabaret, with more Bond gained depending on how much money you spend on the event. However, this is a boy's only event, so Saeko will not attend.
These two events raise a character's Bond, but once they reach of the end of their Bond bar, they will get a lock symbol next to their Bond. This means they can no longer gain Bond, and are ready to level up. Speak to them in the Survive Bar to complete their level up.
How to raise Eri's Bond
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