The two empty slots for espers are supposed to be there: One slot remains empty so you can always unequip an esper if you need to and the second spot is from (SPOILER)when you "upgrade" Odin to Raiden. Odin takes one spot in the list and when you upgrade to Raiden it takes another spot in the list, blanking out Odin's spot in the list.

As far as characters go, there are 14 total characters to be found.

Just remember that empty slots don't necessarily mean that something's missing.