Jason DesJardin and Emily Holmes are agents with IATO.
That acronym stands for International Anti-Terrorist Organization, a secret group obviously dedicated to finding and bringing down those who would use terror as a weapon to get their way.
Interestingly, nowhere in the three adventures that currently exist for DesJardin and Holmes, I could find no reference to the organization. It is only in the blurb at the beginning of the collection explaining who they work for that the acronym and the breakdown are given. Its history is equally left to doubt. What we do learn is that a man named Avery Holmes was a 'Senior Agent' within the group and that the group dealt in 'black-ops' missions.
Also of some interest is the fact that DesJardin is not 'real' last name of the first half of this unofficial partnership. What it was originally is is unknown, back in the days of his teen years when he was a ward of the foster system. He and three others were 'privileged' to be under the 'protection' of Senator Adam Kincaid. Both words in quotes are so highlighted because both are how they are perceived to be but not at all what they are in fact. We are able in the first adventure to follow the evil of the Senator and how Jason had to act to save his foster-siblings. We also see how he came to become the 'nephew' of Avery Holmes which would in later years lead to his joining IATO. For a few years then his last name would be Holmes before it become DesJardin, likely when he joined IATO.
Emily Holmes, as said above, an operative with IATO as well. She came to the group a couple years after DesJardin, being seven years younger. She is following in her father's footsteps as an agent after turning 21, intent on being part of the investigation into the murder of her father a bit before the adventures begin.
The two operatives share in their determination to bring to an end the terrorist group that killed the elder Holmes, though Jason will be constantly concerned with Emily being "too young" to be put into such dangerous positions.
Of course, the fact that he is and has been in love with her for quite a while might also play a part. Luckily she feels the same way for him. Too bad they are both too stubborn to admit it to the other.