Marriage Guidance Counsellor

The Marriage Guidance Counsellor was a serial killer active in the city of Ankh-Morpork for several years immediately prior to the events related in the canonical novel Men At Arms. 
At first the resurgent Watch were perplexed at the unconnected deaths of a figure variably estimated at between eighteen and twenty-four men around the City. (Had they but known it, the killer had already been interviewed and ruled out as a suspect - by Sergeant Colon and Constable Nobbs - right at the start.). The only linking factor, as the Tanty Bugle worked out, was that all the "victims" were men who had bad reputations for offences against women and children, and in most cases were known wife-beaters or child abusers. Or both. 
It took the Guild of Assassins' internal investigators to identify a suspect. The killer, universally acknowledged to be a "she", had in the opinion of the guild carried out her unlicenced inhumations with style and grace worthy of being honoured with retrospective full Guild membership. The fact she already worked for them - as a tutor in etiquette, grace and elocution, to whom the Guild sent remedial cases for improvement - was a factor in Lord Vetinari being prevailed upon to offer an Angel, after her detention. The Guild's veteran lecturer in Inimical Alchemy, Mr Mericet, was instrumental in her identification, a factor for which she held no ill--will afterwards. This is recorded in the story The Graduation Class. 
In fact, now she is a full Assassin and Guild School teacher herself, Joan Sanderson-Reeves is known, to braver pupils, as Mrs Mericet - it has not gone un-noticed that a certain understanding is developing between them. 
And this is where it should have ended. 
But a year or two later, a new Marriage Guidance Counsellor arose. This time, Joan was instrumental in tracking down the woman trading under her own name. And in the fullness of time, another Angel was offered.