Diana Wynne Jones Wiki

Derek Mallory is the adoptive father of protagonist Maree Mallory, and is the person she always refers to as her 'liittle fat dad.'

His brother Ted Mallory is the adoptive father of Nick Mallory, so the family is all connected by these two adoptions which occurred around the same time. Ted is still married to the cold, sour Janine Mallory, but Derek has been divorced by his crunchy granola ex-wife, and has developed cancer, which he is still being plagued by.

He first appears in the novel Deep Secret when protagonist Rupert Venables visits him in his attempts to track down Maree, whose name is one of the five on the list of potential Magids that Rupert is researching. Derek explains Maree's current location, her current university situation, and is in such obvious pain that Rupert tries to tell him the cancer's hanging on at least partly because Derek is hanging on to it.

"Funny, that's just what Maree told me," Derek says, and Rupert leaves him after doing some magical first aid to improve Derek's attitude and inner sense of worth, which seems to make a difference.