Kiros Seagill is a character that you temporarily during random dream sequences that occur at various points of the game. These include:
His group is lead by Laguna Loire and he is accompanied by 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!)
Kiros’s Limit Break is called Blood Pain which has him go in to a blood thirsty frenzy attacking a single opponent.
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.
