Serafina Pekkala is the queen of a northern witch clan, at Lake Enara, a fictional representation of Lake Inari in Finland.
Before the setting of the trilogy (most say 40 years prior) Serafina had a relationship with Farder Coram, and says in NL/GC that she would gladly have stayed with a life as a Gyptian, had two things not occured. The first, was the death of her infant son, whose father was in fact Farder Coram. He died as a result of a disease which swept from the East. Although it is most likely that her son would have died before her, and aged where she did not, as is the nature of the children of witches, (for if the child had been a girl, and not succumbed to the disease it would have been a witch, and may have outlived Serafina in all probability) it most obviously caused a wound that could never be healed. Also, there came the death of Serafina's mother, (who before her was the clan queen of Lake Enara) which meant that Serafina must return to the North, and forever leave Farder Coram, who, as Lyra meets him in the trilogy, is a kindly, old and wise sort of man, with a hidden longing for his lost love.
His Dark Materials
Upon arriving in the north, the Gyptians call upon her service and that of her clan's. Serafina plays an active role throughout the trilogy, and rescues Lyra and company from many tight situations.
Serafina and her witch clan assist in defeating the Tartar regiment at Bolvangar, and rescue Lyra and Will from the marauding children attempting to kill the two in Cittàgazze, and assist in the final climatic battle of the third book.