Belias, Mateus, Adrammelech and Zalera takes one Mist Charge.
Shemhazai, Hashmal, Zeromus, Exodus and Cuchulainn take two Mist Charges.
Chaos, Famfrit, Ultima and Zodiark take three Mist Charges.

Espers are useless. I rather have my Vaan fighting alone than have someone that require me to heal him/her everytime.
The only Esper I like to use is Exodus because he's a Libran.

But I'm a completist, getting them all is a must.