Grandpa is utterly devoted to his grandchildren and takes a huge interest in everything they do. Everyone in the Mason family, except Jason, is very puzzled by Grandpa's attachment to his tatty old flat cap, which they never see him wear. When nobody else is around Grandpa puts the cap on and shrinks and suddenly, all his usual aches and pains vanish and he becomes extremely agile and athletic - climbing onto the table, leaping from shelf to shelf, jumping off high places. Jason chases him around the house to Grandpa's playful cries of "Catch me if you can!"
Jason is seven years old and spends a lot of time with Grandpa. He is a smart kid with a lively imagination. Jason has inherited his dad's interest in making and fixing things and loves all his toy vehicles and machines. He has a good sense of humour and - in spite of the fact that he often has to sort out problems caused by Grandpa - he can always see the funny side. Jason narrates every story, so the situations are seen through his eyes.
Jemima is Jason's ten-year-old sister. She loves singing, dancing and acting and has hopes of being a movie star or a famous pop singer. Jemima is constantly practising her moves, especially in front of Grandpa who is a captive audience. She can be a bit of a drama queen, with a tendency to over-react to situations, but she is entirely charming and lovable. She spends hours choosing her clothes whenever she's going anywhere and is always monopolising the bathroom and experimenting with her hair. Jemima gets on pretty well with Jason, considering he's a younger brother, although they do have their moments of conflict. Jemima adores Grandpa, but has no idea about his shrinking abilities.
Julie is Jason and Jemima's mum and is utterly dotty and delightful. She is enthusiastic and is always taking up new hobbies - whether it's cake decorating or karate. Mum never worries, panics or gets cross - she stays completely calm and greets every bizarre thing that happens to the family with total composure. She's very grateful to Grandpa for looking after Jason while she's working with Dad in the family bike shop and wishes Grandpa were fit enough to get about more.
Dad CJ runs the bike shop in town which is called 'Lend it, Mend it, and Vend it'. He is fantastic at fixing bikes, but not so good at fixing anything else. Dad is quiet, cheerful and relaxed with a special love of sandwiches. He likes to spend time down at the family beach hut or cycling around Sunnysands, away from the chaos of life at home. Dad is often mystified by the strange goings-on in the Mason household, but he can never come up with an explanation. Beowulf is Grandpa's scruffy old, inappropriately named Yorkshire Terrier. He is a cute and cuddly dog and spends most of his time sitting on Grandpa's lap. Beowulf can't talk, but he has a mischievous side and can complicate an already tricky situation.
Loretta is Grandpa's bossy, eccentric and very temperamental sister whom he finds deeply annoying. She is fussy and very critical and rarely sees the funny side of things. Great Aunt Loretta has a very odd dress sense and she also brings her own odd combinations of food when she comes to visit, such as Spinach and Sprout Shake. Great Aunt Loretta has the unfortunate habit of stubbing her toe whenever she comes to visit and the family keep a packet of frozen peas in the freezer especially for these moments. Whatever the event, Loretta always tries to take over - something that Grandpa resists vehemently.
Miss Smiley runs the café on the pier where she serves wonderful ice-cream sundaes and delicious homemade cakes. She is very kind and very friendly and Grandpa is her biggest fan. Miss Smiley's nemesis is Mr Krumpgrumble, the taste tester at the town hall who is extremely hard to please.
Troy lives next door to Jason and he's always inviting himself round to play. He always has the latest toy or gadget which he likes to show off to Jason. Troy is used to getting his own way, he has bad manners and loves scaring people or playing tricks on them. Unsurprisingly, Grandpa doesn't like Troy very much and sets out to teach the boy a lesson. Troy has encountered Grandpa in his miniature form a few times, but always when Grandpa's in disguise.
Mr Whoops is the charming, rosy-cheeked toy shop owner in Sunnysands. He's called Mr Whoops because he's always having little accidents. Grandpa loves to go to Mr Whoops' toy shop with Jason, especially when he's got his shrinking cap on, as there's so much opportunity for mischievous fun!
Mr Liker Biker is a frequent visitor to Mum and Dad's bike shop and often helps out in the shop when Mum and Dad have to go away. He is obsessed with cycling gear and equipment but is always so busy choosing and buying it that he never has time to ride his bike! Kind-hearted but a bit of a ditherer, Mr Liker Biker often needs Grandpa's help to spring him into action.