How old is she?Although OotP gave us some hints which helped us determine how old Bellatrix is, there is still no concrete proof of Bellatrix's age. This is an important part of the puzzle as many of the other mysteries depend upon the answer to this question.
How did she survive for so long in Azkaban?This is certainly a tough question to answer. Some people believe she survived because she was already "insane" before she was placed in Azkaban (refer to question 4) and she was therefore not at any risk of loosing her mind. However, those of us who believe that she was still of relatively sound mind before being placed in Azkaban, which brings about the mystery in question. We have really only gotten a first hand account of Azkaban from Sirius Black. He funnily enough has a similar temperament to Bellatrix. For example, they are both extremely determined, very loyal, and are both passionate about the causes they support. This would suggest that they both survived in Azkaban using similar mechanisms, however, there is still a notable difference. Sirius stated that when the Dementors wore him down emotionally, in order to keep his mind he would transform into his animagi form, a dog, who has simpler emotions and therefore doesn't get as badly affected. As far as we know, however, Bellatrix is not an animagus and would not have had this ability. How then, did she survive in Azkaban for such a long time?
Does Bellatrix have any children?There is the possibility, given her assumed age, that she and her husband would have been able to have children in the time between Bellatrix leaving school and Voldemort's rise to power. If she did have children though there are some obvious questions that arise. Where are they now? Who has looked after them? And will they appear in future books?
Is Bellatrix "insane"?This is a very interesting question which prompts a lot of different view points. From a strictly psychological point of view, Bellatrix is not "insane" in a clinical sense. Clinically, to be "insane" a person has to display signs that they did not know what they are doing or is incapable of understanding that what they are doing is wrong. This last point seems to suggest that Bella is, in fact, not clinically "insane" by her statements to Harry within the Department of Mysteries. Even so, she seems to display characteristics of psychopathy. Psychopathy is described as a mental disorder with an emphasis on interpersonal traits such as pathological lying, egocentricity, lack of remorse, and callousness. This, however, is still questionable considering her devotion and ultimate obedience to Voldemort. On the other hand, a social psychological view point is suggestive simply of an authoritatian personanality type (this is not classified as a mental disorder) which is indicated by a respect for and deference to authority, an obsession with rank and status, a tendency to displace anger and resentment onto weaker others, an intolerance of uncertainty, and a need for a rigidly defined world. All traits which Bellatrix herself displays. Really, only a thorough psychological assessment of Bellatrix would answer this question properly and as this has not happened, as is not likely to happen, Bella's soundness of mind remains a mystery.
How did she manage to get trained by Voldemort and get into the inner circle?Another mystery that is extremely hard to form any opinions about. Some believe that she was hand picked by Voldemort when she was still at school and she began to be taught the Dark Arts by him during holidays and as soon as she graduated. Some are more skeptical, however, questioning why Voldemort would have hand-picked an underage female wizard to personally train. Additionally (and adding to the mystery) she appears to be the only Death Eater within Voldemort's inner circle who is female. There must be something about this woman to have made her such an appealing choice to Voldemort. But what is it?
What has happened to her parents?As with Sirius Black's parents, most people believe that Bella's parents have also passed away. Nothing has ever been said for certain however, so this is still considered a mystery.