As a pternodon, Henry aka Morgan le Fay saved Jack and Annie from the T.rex in Dinosaurs Before Dark.
She's more of a Minor Character but appears in most books telling her kids to come back at a certain time and at the end calling out, "Ja-ack! An-nie!"
T.rex
Jack saw the dinosaur in Dinosaurs Before Dark. Henry, the pternodon, saved Jack and Annie from the T.ex.
Kama
In High Tide In Hawaii, Kama appeared. According to Jack, she does not have the ability to "read" but she does have the ability to surf. This is the last book that is not a Merlin Mission.
He is a pirate in Pirates Past Noon that is the captain. He appears to have a dislike for books.
Pinky & Stinky
They are pirates in Pirates Past Noon and the servants of Captain Bones. Stinky may actually appear to be stinky, due to his name being Stinky.
Teddy is more of a main and minor character, due to the fact he appeared in books 17-20 as a dog. He later appeared in book 30, the second Merlin Mission. He has accidently turned himself into a dog after stealing Morgan's spell book and using it. He is also the helper of Morgan from Camelot.
Tiger
The tiger appeared in Tigers Before Twilight and had his front paw caught in a trap. Luckily, Annie freed him causing the tiger to charge the three. Jack and Annie had Teddy as a dog in Tigers Before Twilight.
Soothsayer
The soothsayer that appears in Vacation Under The Valcano is an old lady who appears twice in Vacations Under The Valcano. She is speechless when first met by the two but later she explains Pompeii will no longer exist the next day, due to the valcano erupting on the day Jack and Annie arrived.
Peanut is Morgan le Fay. Merlin likes to pull tricks on Morgan and the first one was turning Morgan into a mouse named Peanut. The spell is broken after books 5-8 where Jack and Annie find a mouse, moonstone, mammoth bone, and mango.
Dolphins
The dolphins appeared in Dolphins Before Daybreak, where the siblings solve a first riddle and try to solve three more to become Master Librarians.