Catherine (her true name has since been lost to the ages) was born a mere peasant girl in a tiny, quaint village in medieval France, on an Earth not too unlike our own. Her long journey to greatness, however, started at only a few years of age, when war saw the village sacked and burned, leaving only a handful of scattered survivors. She herself was taken in and given a new name by a local coven of witches, but she didn't stay long with them; under their tutelage, she learned to read and write frighteningly quickly, and as they taught her their craft, she was constantly inquisitive, questioning everything, dissecting the answers with further "How?"s and "Why?"s, and recording everything in a journal, cross-referencing what she learned for inconsistencies.
After only roughly a year, she had learned all that the witches could teach her- or, perhaps, they became fed up with her insatiable thirst for knowledge- and she left them, wandering east as an orphan, with naught much more than a traveling cloak and her journal tucked under her arm, sustaining her travels and experiments by plying her skills as an apothecary, asking only to learn what the people she met could teach. Naturally, a seven year old girl with the calculating demeanor of an adult woman, traveling the countryside alone, and healing the sick for naught but food and lodging had a mixed reception with the superstitious peasantry; some towns ran her out as the witch she was, while others lauded her as a living saint- as she took care not to make public her dim views on religion, pagan or Catholic. Either way, word spread quickly as she approached the Holy Roman Empire.
Naturally, being something of a local celebrity, and a child perceived to be too weak to defend herself, many lords around the region sought to make her their guest of honor- even as she went to greater lengths to conceal her identity- through any manner of bribes or thinly-veiled threats of force. In some cases, she acquiesced, if for no other reason than to peruse their libraries- but she never stayed in one place for more than a year at a time, even if it meant escaping under the cover of darkness, using her rapidly expanding repertoire of languages, hexes and alchemical substances.
This finally came to a head on the same day as the first time she achieved an overt display of magical power- and the first time she killed someone. During court with the King of Austria, an especially brazen assassin tackled her and attempted to take her life, and, out of fear, in front of God and everyone, she grabbed his face and set his head aflame, practically shouting the incantation in the experimental magical language she had been developing. Absolute pandemonium ensued, and she managed to escape into the streets amid the chaos; from there heading south towards the Mediterranean- which turned out not to be the wisest move, as Rome decided that, after such a violent display of heathen sorcery, she was better off dead.
In Venice, at the age of 14, she finally got the break she needed: Catherine, the White Witch, died... publicly. A secret society of Hermetic occultists took their chance to contact her in her hour of need, and were all too eager to stage her death and spirit her away to their stronghold in Alexandria; and, finally having access to a steady supply of material to research and experiment with, and a group of peers with which to trade findings with, she was more than happy to drop out of the public eye... though even the Hermeticists had to bite their tongues at the way she rigorously assaulted their doctrines, stripping away what she considered irrelevant religious connotations to get at and scientifically test the flesh of their mysteries. For almost a decade, they stayed on... relatively good terms, Catherine frequently traveling between Egypt and the Italian peninsula as she pursued her studies with seemingly bottomless zeal; it was during this period she took on Knowledge as her second name- after her one absolute, driving goal.
Over time, she slowly drifted away from the Order, as she found their help gradually less useful, eventually completely seceding from it, and going on to found her own secret society for the pursuit of arcane lore; and, at the age of only 23, she finally achieved the Holy Grail of alchemy: the creation of the legendary Philosopher's Stone, and, through its transformative properties, the synthesis of the Elixir of Life: combined with her steadily growing magical might, she achieved functional immortality, no longer needing to eat, drink, sleep, or age.
Thus, her exploration of the great mysteries of the universe would continue for some time; as years turned into decades, and decades wound into centuries, history continued much as was planned- though the Renaissance and its rapid technological growth would capture Catherine's heart much as her other obsessions had: she would work with her world's Leonardo da Vinci himself, inducting him and other rising geniuses into her order, and, with selective application of the Elixir, they would go on to collaborate for centuries to come.
Unfortunately, as grand-mistress, she failed to really keep abreast of worldly events, and where things would eventually accelerate beyond the grasp of even a potent order of magicians would be when the second World War finally erupted in the 20th century, and extraterrestrial powers got themselves involved: When the Order discovered that Adolf Hitler was, in fact, an alien plant, and the Martian Empire was behind the formation of the Third Reich, they were quick to side with the Allied forces- even though they didn't mention such, to avoid a global panic. All pretense of subtlety would soon be lost however, as the paranormal elements of each became increasingly involved in the war, and the already chaotic situation would become downright apocalyptic when frequent Shiftspace use by human researchers and explorers from neighboring realities finally caught the attention of the ancient harbinger fleets of a power that would consume their entire dimension; The Abomination.
Even the virtually magical technology of the massive Martian fleet was like naught but wheat before the scythe of the Vanguard, and Catherine's Order, the Allies, and the Martian Axis came to a hasty truce; when the rapid response fleet from the Pan-Dimensional Alliance of Humanities arrived, they worked to coordinate a planetary evacuation as best they could with their dwindling forces- Catherine herself pledging the Alas Aurum, her own experimental spacecraft, to the effort- before fleeing into Shiftspace; She was recommended for the Heroes program for her valiant efforts and tenacity throughout history, as well as being that particular Earth's most powerful sorceress- and, in fact, when she learned of the program, she volunteered, both to learn about these aliens and this Abomination she was hearing about, and to take revenge for the destruction they'd wrought.
Despite being over 500 years old, Catherine is a youthful looking woman of blue eyes and a relatively average build (for her time, at least)- though it can be difficult to tell through the voluminous silver robes, gilded in gold and violet, she wears almost perpetually, and an almost completely sedimentary lifestyle has left her quite scrawny- similarly, her skin is quite pale from a lack of sunlight, and her hair, reaching down to her knees when not contained within her hood, has long since been bleached white by time and magic.
Though she does have dry sense of humor, she tends to be cold, analytical, and aloof, far more interested in her personal projects than socializing, except insofar as that socializing involves her work, or debate of theology or philosophy, and she can usually be found with her nose buried in some manner of ancient grimoire, now that the libraries of whole different galaxies and dimensions have opened up to her- literally, as she's nearsighted (she does possess glasses, and could probably repair her eyes with magic, but as she's usually perusing exotic tomes or tinkering with her myriad gadgets, these are infrequently used/a low priority that she hasn't gotten around to, respectively).
Psychic: Catherine is an exceptionally potent magician (a term which she considers more dignified, and not to be confused with stage illusionists and other charlatans), with around 500 years of study and practical experience: many of her arcane abilities have developed to the point where they only require a single, simple gesture or word, or even neither, being easily confused with actual psychic powers- she eschewed expending the effort of walking long ago in favor of simply levitating where she wants to go. For more powerful and complex incantations, she possesses a massive, leather-bound, and heavily enchanted grimoire, something like the hundredth iteration from her original journal of herblore and witchcraft, and her Philosopher's Stone; an artifact capable of altering the physical properties of any material- the transmutation of lead into gold is but one facet of its power, and, utilized properly, it can prolong life indefinitely, or even restore life to the dead.
Shield: While she works with technology extensively, and is constantly surrounded by a swarm of tiny mechanical drones imbued with magic and suited for various tasks, Catherine's alchemy and gadgetry tends to focus on creation rather than destruction, and takes on a more supportive role, best characterized by her companion: Alexander, a machine built in collaboration with da Vinci resembling a giant brass lobster, once an experimental submarine, but that has, since being rendered obsolete, been repurposed into a mobile library and laboratory: it's back unfolds to reveal a comfortable chair and a dizzying array of tools and apparatus, as well as containing bookshelves, a secret compartment for storing curios collected over the centuries, and a Famicom and small screen, plugged into the machine's magical power source, for the extremely rare moments Catherine allows herself a moment of spare time. While it can follow simple instructions, and can still comfortably house a person in a safe, airtight environment while collapsed, however, the machine is bulky, slow, and weak, both in armature strength and in armor strength against super-modern weaponry; essentially useless in a fight.
When not simply blasting her enemies with her substantial arcane might, Catherine's weapon of choice is a specially enchanted musket with a slightly flared barrel- rapping it solidly on the side and speaking a command word cleans and primes the chamber, and she uses it both to fire an array of specially enchanted ammunition and as a tool for launching drones and alchemical compounds into the fray.