|Age||not given; about 16|
|Occupation||Witch, sort of...and Lancrastian Ambassador|
|Residence||originally The Shires, then Lancre|
|Parents||Lord Harold and Lady Swivel|
|Relatives||b. Harry, b. Hugh|
|Books||The Shepherd's Crown|
Geoffrey becomes not-quite-a-Witch, capable of most of the same functions but relying on a sort of headology based on persuasion rather than Boffo and fear. He has a rather eldritch goat as a companion and protector rather than Esk's staff. (The concept of a familiar hasn't been featured previously in the Discworld chronicles, unless you count Greebo's occasional assistance.)
Geoffrey's father, Lord Harold Swivel, has managed to revive a family fortune almost dissipated by his father and establish himself in a large country estate in The Shires. He has no respect for education but employs a tutor for his three sons at his wife's behest. Geoffrey, the youngest, is the only one to take an interest and reads prodigiously. When his father dismisses the tutor, he turns to McTavish, an old family retainer in the stables, for education in practical and natural areas.
Geoffrey's fondness for animals, starting with Mephistopheles and nurtured by McTavish, finally clashes with his father's values in a fox-hunting episode and he decides to leave home. He has read about Witches and seen them on broomsticks in the sky and decides that this is his calling. Narrative Causality draws him to Lancre and Granny Weatherwax's cottage. Tiffany passes on Granny Weatherwax's broom to him, after the Dwarfs completely replace everything. He shows quite a facility for broomstick riding, even being able to break the sound barrier, which, on the Discworld, crowds lightspeed as well.
He arrives just in time to face a massive attack by the Elves on two fronts and he and Mephistopheles acquit themselves well. He has shown himself capable in the practical elements of Witchcraft and is able to relieve Tiffany Aching of the second steading in Bad Ass which has been imposed on her. He impresses King Verence and Queen Magrat enough that they appoint him Royal Ambassador to the Sto Plains and he is empowered to return home and kick a little sand in his father's face. (Mephistopheles takes the opportunity to plant a kick in the other end.)
There is no mention of a relationship to Tuppence Swivel. If the very literary Tuppence were part of the family he would be considered a black sheep by Harold.