Katherine was the daughter of Kallak, the leader of the Royal Mystics. She fell in love with and married King William and they had a son together, Brandon. The family were very happy and close for a time, in book three Malcolm recalls William taking Katherine on picnics and reciting poetry to her. Their happiness and lives were violently cut short when the Royal couple were stabbed to death. Their infant son survived and was raised by Kallak.
Malcolm was believed to have been the murderer as he was caught with the bloody knife and imprisoned for 18 years.
Unlike her husband, Katherine's ghost can be summoned by placing flowers on her grave.
When Brandon summons her in the first game she immediately recognises him, despite the fact he was an infant when she last saw him, and is happy to see him. Brandon wants revenge against Malcolm, but Katherine calmly advises him not to waste his powers on destruction and to control his anger. She says that he needs to free the Kyragem to retrieve his throne, and then activates the invisibility Gem on his amulet, before telling Brandon to be brave and use his powers wisely
In Malcolms revenge we learn that Malcolm was not guilty of murder as previously thought, but that while William knew the truth, Katherine along with everyone else believed Malcolm had murdered her and William, and she greets Malcolm very coldly when he summons her ghost, calling him a murderer and telling him he has a nerve to summon her.
However when Malcolm explains (in nice mode) she does seem willing to listen, unlike most others, and although maybe not fully convinced, she does tell Malcolm that he needs to perform a royal seance to summon Williams ghost when Malcolm claims William can prove his innocence, and her voice loses some of its hostility before she disappears..
This game also seems to indicate Katherine was murdered shortly after William as William says they (he and Malcolm) were alone in the dining hall when the enchanted knife first attacked, and Malcolm grabbed it to try and stop it. This would explain why only William knew the truth.