David Avivi is an agent with Mossad.
Though he is still a relatively young man when we first meet him, his experience in that intelligence agency is considerable. So much so that he is in line for, and will shortly receive a promotion to Deputy Chief. It does not take long when reading how many officials in other countries' clandestine services treat him, all very friendly and respectful, to see that the promotion will be based entirely on merit. When it comes to fighting terrorism, Avivi knows what he is talking about.
If his qualifications in counter-terrorism were not enough, Avivi had just recently as of the first recorded adventure received a doctorate from the Weizmann Institute of Science. Interestingly, the region of study was not at all related to his day job but rather in the field of physics.
We will learn over time that Avivi came from a "family of scientists who had worked in two of the renowned Israeli government research institutes". His father had a doctorate in chemistry and his mother hade one in physics. Both parents had taken their expertise to very high levels in the practical, real-world arenas. Along the way they passed on their appreciation of science to their son and they remain sources of good advice.
Considering, though, many of the situations that he will find himself deeply involved in, problems brought about by the terrorists he fights will involve nuclear fuel and nuclear explosives. Having someone like Avivi be reasonably versed in what can go wrong is highly useful. Having Avivi physically trained and prepared to take action is also good.