Evie and Andew Walls are students.
They are not spies, at least not when we first meet them. Their mother is, though we are not supposed to know that at the beginning of the first recorded adventure and certainly Evie and Andrew do not. They are 11 year old twins. Actually just turning 11 when we are introduced to them, born on the 11th day of the 11th month, to be more informative. On that day, they were expecting all sorts of good stuff happening, a Day of Unbelivable Importance, it being their birthday and all. That would turn out to be far truer than they could have imagined.
As mentioned, the Walls are twins. Evie is the older by far - 3:43 am to his 3:47 am - which naturally made her "smarter, more mature, and more organized". Andrew would modify that slightly to just say she was more bossy..
Okay, so the thing that takes these two pretty normal tweens and forces them to become spies is their mother - or more accurately the lack thereof. As the two raced downstairs into the kitchen expected a great breakfast, they found instead "a torn envelope address to both of them" laying on the table. It was from Mom. Dad, it turn out, had one as well. She wrote that she regretted the fact that she had to leave so abruptly and that they would all celebrate the birthday whenever she got back but that might not be for a long time. It also contained a cryptic valediction with the urge to remember: "11 11 11: A picture forms without you - and it's all a jumble".
Things really start happening for these two a year later; a year spent without their Mom and no idea when she might be coming back. A year in which Dad's covert work for the military caused a transfer to yet again a new town. Everything was new and different and exciting and scary. Then the first mysterious package arrived on their doorstep filled with random items and instructions to do strange things.
Which leads us to Spy X, the name given for this set of adventures and used inside them to denote the person they believe left that package and subsequent ones, all giving information into their mother and what they need to do next. What they do not know, and I am certain Spy X did not know either, was that soon Spy X would be in great need of their help.