Who is Colum MacKenzie from Dragonfly in Amber and what is their importance?

Colum MacKenzie is Jaime's uncle, and the laird of castle Leoch. He suffers from a disease that causes disintegration of the bones, which over time, has caused his legs to shrivel and become useless. Colum governs castle Loch with the help of his brother Dougal. He is married to Letitia, Hamish is their son.