You can't fire him, he's essential for the game's plot. He will leave you later of his own accord.You can access your own characters by pressing one of 1, 2, 3, or 4. The first press selects that character as highlighted, and another press accesses the character screen.
Similarly, pressing 5, 6, or 7 accesses the NPCs. Pressing 5 (and pressing again if necessary) opens a dialogue window with Forad Darre, whom you can talk to but not fire. Pressing 6 or 7 will enter your own NPC's dialogue boxes, and there should be a dialogue there which gives you the option to fire them.
