Mombi was a witch posing as an apothecary, living in a town with no name. She kept Tip 'captive' in her house.
In the world of the Emerald City
Mombi acted as an over protective mother towards a young boy named Tip, whom she kept in-house all time.
She pretended it was for tip's own good as he was of ill health and required special medicine each day.
That medicine was in fact a dark elixir that suppressed Tip's true nature and kept him/her in a boy's form.