Dad

Dad is a nickname given to Stephen Reyes by the junior members of the ACTF.

Chronologically, the first reference to Reyes as "Dad" occurs in episode 1x01, "Breathe", as Daphne Worth and Nicolette Lau wait for the rest of the team to come to the briefing room:
 * Lau shook her head and put the grin back on. "Mom and Dad using the serious voices. It'd be wrong not to at least try to eavesdrop." She tipped her head toward the chair at the whiteboard end of the table. "Reyes gets to watch the door. Otherwise it's general admission."