X of the Underground is a spymaster.
She earns that distinction as her role as the leader of a Resistance group.
The time period of her activity is the early 1940s. The location is France, mostly the Paris region. The circumstance is the occupation of most of France by the invading Germans and her fight against the Nazi control.
She is almost certainly of French nationality as I indicate in the fact sheet but that is an educated guess based on her area of operation and the fact that she heads a group of women who also seem to be French citizens.
Her belonging in this compendium does not come from her leadership in this Resistance squad, admirable as that is, but in the fact that she takes her defiance to levels far outside the 'normal'. She does not limit her activities to just attacking the enemy when able but also goes to great length to infiltrate their command centers as needed to accomplish her goals. This includes assuming disguises and taking the identity of the enemy, actions which are clearly denoted as those of a spy and punishable by imprisonment or death.
The identity of X, or Madame X as she is sometimes addressed in a more formal manner, is unknown. She is likely someone in her late 20s or early 30s but that, as before, is speculation. Certainly her penchant for appearing in one disguise or another makes it very difficult to gauge. She wears what we could consider her natural dark hair cut very short, almost certainly to make it far easier to don those disguises.
Speaking of which, we first meet her when she is in the role a German major interrupting the firing squad execution of several Frenchmen. While she appears in the pictures we have of her doing this to be female, the response of the soldiers she forces her will and her rank on do not seem to suspect either her impersonation or her gender. A German officer she accosted before stealing his clothes definitely referred to her as "he". This shows her ability to hide her gender both physically and audibly as well as to speak German like a native.
X shows herself to be every bit as gutsy and ruthless as her position demands. In the first adventure she not only dressed as a German soldier, she also donned a suit and tie and showed up at Gestapo headquarters as a male reporter. In the second adventure, after evading capture as an old woman, she is later betrayed by a jealous team member and nearly arrested by two German officers. Both men and the traitorous woman were found hanged at her command and likely assistance.
Also a part of the recorded adventures we have of X is the 'meddling idiot', her words, known as Bob Gray. That man is an American reported operating in Europe looking for a good story to interest the readers of his employer, the New York Globe. While he is enchanted with her as well as wanting a great story, she is more likely to be vexed to see him ("You, again!) than pleased. Also she seems to have trouble remembering his first name - a couple of times the narrator will refer to him as Bob and he introduces himself as Bob but in at least two adventures, she calls him Jimmy. It does not seem like a romance destined to be. He is not even in the last adventure.