EXP system is quite simple. You can roughly divide the Leveling system to 2 components. The Skill Level which has its own calculation and individual protagonist level.
EXP requirements increase 100 per level for both systems.
Status points increase 1 gain per 10 levels and are always consistent example:
Level 1-9 gets 1 point per level
Level 10-19 gets 2 points per level
ETC...
When it comes to skill points its actually more complicated but simple after you figure this out.
The skill status keep track of its own level per say. It's easier to give an example.
Let's say you mainly play as 1 character and that one character is
level 10 and you start playing as the secondary protagonist. For you to get an increase in skill points it would require for your obtain 1000 exp on the other character which is the same as going from level 10 to 11 on the initial protagonist you obtained level 10.
Skill points increase 1 gain per 10 levels and are always consistent example:
Level 1-9 gets 1 point per level
Level 10-19 gets 2 points per level
ETC...
In essence, for obtaining skill points, its probably more efficient to raise only 1 character because that one character would have an easier time to get the needed XP to raise your skill points level
PS: The status gain system also applies for every other NPC.
John8r9 I figured out how the skill points system worked idk if I made it easy to understand. It's easier than I thought it would be I should've tested this out earlier to understand how leveling works in this game