Laguna Loire is a character that you temporarily play during random dream sequences that occur at various points of the game. These include:
He is full of energy but is also extremely clumsy; during the sequence at the Centra Excavation Site he will repeatedly drop an Old Key that he finds in the various corridors.
He is the leader of his team of three which also includes Kiros Seagill and Ward Zabac. The three of them face each of these adventures together and the story eventually converges with that of the main party (no spoilers here though!)
Laguna’s Limit Break is called Desperado and it involves him throwing a hand grenade and then peppering multiple enemies with shots from his Machine Gun.
Similar to the other temporary party members that join your team, you will only have control of Laguna, Kiros and Ward for a short amount of time. However, because your characters are chosen as stand ins, Laguna/Kiros/Ward will have your current party’s junctions and magic, which means that any magic you 
Draw from enemies will carry over back to your original party.
Leveling up Laguna, Kiros and Ward serves no purpose in the game though so you should seek to complete the Laguna Dream Sequence sections as quickly as possible.
