Aforementioned caveats about being busy as, but this is too cool to ignore, so:
Character:
Inquisitor Kerralyn Merriweather
History:
Age had turned Inquisitor Ibrahim Ivanovych Sammael to all manner of radicalisms and heresies, and although his Istavaanian leanings made the concept of proteges abhorrent, he still thought he could pick and nurture more suitable inquisitors than "the usual procession of puritan dotards". So, in his fourth century of life, he resolved to look among the nearby schola progenium for their most able candidate, and take them 'under his wing'. He alighted on one girl in early adolescence: extremely precocious, arrogant, and borderline insubordinate. Her name was Kerralyn Merriweather.
Sammael proved an eccentric mentor. Most of Merriweather's tutelage consisted of scholarship (Sammael gave her free access to his library, with no attempt made to protect her from the books) and debate (routinely him playing 'daemon's advocate' for all manner of heresies - he concealed the fact that many of these he sincerely believed). The occasional field-work and combat were uneventful successes. Sammael hoped that a young mind, not normalized to usual presumptions, prescriptions, and taboos would rapidly gravitate to some interesting radicalism. He was disappointed: Merriweather's researches, although sufficient to have her purged by Sammael's peers, became more cautious; and although Sammael generally won their arguments, she remained a (deeply conflicted) Amalathian. Despite this, Sammael respected and valued her company - he found competent ideological foes a rare commodity.
True to form, Sammael contrived to have Merriweather raised to full inquisitor half way through her third decade - considered by most outrageously young, but Sammael reasoned he had nothing left to teach, that she would survive if she was good enough, and it would make an interesting experiment. He was a member of a loose diaspora of radicals, united by little more than contempt for inquisitorial orthadoxy, and few favours were needed to find those needed to ratify. Sammael left Merriweather on the nearest hive with a seal, a considerable amount of money, and a promise he'd check up on her.
Merriweather was grossly inexperienced, and her (relative) orthadoxy put her out of set with the only inquisitors she knew. She began carefully, slowly gathering a core of staff and network of safe-houses and associations in-system, financing herself through a mix of shrewd investment and corruption. She showed a knack for 'arms length' intervention, seldom interacting with but manipulating the artifice of Imperial power to put down low-level rebellions and occasional cults. She attended her first conclave a few years later, and although considered a curio by the radicals and a reckless aberration by the puritans, she earned some measure of respect from both for her intelligent remarks in counsel and reports of her elegant case-work.
Sammael and Vyzier had a tangential relationship of contempt tinged with mutual respect. He was one of those she black-mailed with past heresies to elevate Erich. Although Sammael might well have done so without being threatened, his injured pride led him to try and concoct post-mortem revenge. He had kept enough tabs on Vyzier to hear of her purge around the planet, and that it's warpstorm had ended. Sammael surmised whatever was going on was important to Vyzier, and, better he suspected whatever it was could be exploited for his own ascension. However, he could not act without triggering Vyzier's threat. Yet his virtually unknown student could.
He tracked down Merriweather in the same system he left her. He told her of the planet's history, and whatever other information he could glean to persuade her of it's importance. Merriweather agreed, realizing she should do more than police a system, and booked passage via a series of traders to investigate what has happened. Sammael resolved to follow her after a couple of years - long enough for Vyzier's threats to have passed, but hopefully short enough to ensure the situation was resolved to his advantage. If Merriweather perished, too bad; if not, he was sure he could overcome her if necessary when he arrived himself.
Now:
Merriweather is a tall, white, brown haired women between youth and middle age. Her dress is nondescript, an electoo on her left hand the only mark of her involvement with the inquisition. She is quiet, determined, ruthless, and ferociously intelligent. Without any anchoring in her formative years of what she should think, her beliefs are a syncretic mix of blasphemies: agnostic regarding the emperor, considering chaos as mistaken philosophy rather than profound evil, and others. Her in-practice tendency to amalathianism and protection of imperial structures, as well as her secrecy, help conceal these in-principle heresies.
She is a reasonable duellist and a mediocre markswoman, meticulously trained but broadly untested, and tries to compensate with the quality of her wargear (power rapier, conversion field, etc). However, she considers an Inquisitor being drawn into personal combat a sign of failure, and prefers stealth, espionage, information superiority and other projections of 'soft power', with direct action performed by manipulated intermediaries.
Merriweather is a latent psyker, and although her faculty was beneath the contempt of the black ships, Sammael's reading material has enhanced it without her knowledge (although not without his). It currently manifests as no more than the occasional telepathic hunches, and Merriweather only has limited insight as to what has happened to her.
Obviously, she can have some random agents/redshirts/whatever too. Please check if you don't mind how I've written them in re. Vyzier etc. (I'm happy to change fiddle as you see fit). Actually, that applies to any other recommendations have to improve character and back-story.
Sammael can be introduced later if you like (or not) - as hinted at, he'd be much more exploity than investigatey, and of a much higher power level.
