There will be over sixty missions and they will come in A, B, C, D rank. Apparently it will take a full day of playing to finish one tier on one character, so it could take a while to unlock all the characters Master Role achievements. Some accessories are only available as mission rewards. Read this post here if you want to know more. Thanks to Akatski_Milan for the post.
Assault Ruby
Master the Attacker Role
Destruction Sapphire
Master the Blaster Role
Guardian Topaz
Master the Defender Role
Hex Jade
Master the Jammer Role
Inspiration Amethyst
Master the Enhancer Role
Healing Emerald
Master the Healer Role
Limit Breaker
Deal over 100,000 damage to an enemy
Behemoth Hunter
Slay a wandering behemoth
Perfect Diamond
Learn all abilities to the fullest
Collector of All Things
Collect all weapons and accessories
The All Knowing Sage
Gain the complete knowledge of 100 different enemies
Secret Achievement x 17
Continue playing to unlock this secret achievement
I guess that's cool because a lot of times you have to beat a certain boss to get one of those, but I just hope it's not like a "Seriously 2.0" achievement.