In the Canon:-
Steffi is referenced once, in Thief of Time, as a contemporary of Lobsang "Newgate" Ludd at the Thieves' Guild. Like Lobsang, Steffi was an orphan foundling raised by the Guild from a baby abandoned on the Guild steps, who passed through the Thieves' Guild School with him. In his opinion, she could climb faster and further than any boy of heer age, and sher'd have the purse from your belt without you even knowing.
In the Pessimal Discworld:-Edit
Steffi, like Lobsang, took her name from a place of reverence for Thieves, the city Gibbet where many a thief has been executed over the years. Although she wishes it could have been given a more romantic name, like Tyburn. Stefi Tyburn would be completely her.
She had an adolescent romance with the young Newgate and misses him, although in later years she has come to realise her gender preferences are not orthodox ones. A graduate of the Thieves' Guild School, she is currently employed there as Edificeering teacher. This has brought her into close professional contact with Alice Band, her opposite number at the Assassins' Guild School, who is definitely a woman of alternate gender preferences. They are now also in close personal contact, although it is not an exclusive relationship. Steffi is cool about this, preferring one-half or even one-third of Alice's affections to none at all. Besides, it frees her up to see other women. Steffi remains an ace Thief and Mr Boggis regards her with pride as one of his young stars, who is always ready to take on that difficult bespoke theft. The pupils respect and adore her, and she is a popular teacher at the Guild, always approachable with difficult personal issues.