Joshua
Much like Jill, Joshua is another important character, and while you don’t really get to control him, there is a brief sequence early on in the prologue where you can control him during some battles. This sequence takes place during the 
Flight of the Fledgling Main Scenario Chapter, and while you do get to control him briefly, this sequence is straightforward. But enjoy launching a few massive fireballs!
You also get an opportunity to play as Joshua’s fully-primed Phoenix during the Fire in the Sky Main Scenario Quest. Check out the section below to learn more:
Joshua will also join your party permanently after you’ve completed the 
Where There’s a Will Optional Quest. Check out the link below for more information on when and how to complete it:
Where There’s a Will
Optional Quest
“Joshua has received a message from Cyril, saying that something akin to their father’s last will and testament has been found. He suggests that he and Clive make for Tabor to learn more.”
While in your party, Joshua will launch attacks and cast shields that temporarily protect Clive from damage. His most significant benefit, though, is that he will tie up one or more enemies, which will help you deal with large groups of enemies much more easily.
In-Game Description
*Spoiler Alert*
“Dominant of the Phoenix and younger brother of Clive. His researches into Ultima culminate in the revelation that the creature’s true form is one and the same with Ifrit Risen, the creature he and his brother transformed into in the skies above the Crystalline Dominion. His grip upon the creature he sealed away inside himself by the altar of Drake’s Head ever fading, he swears to stand with Clive and fight their fate until the end.”