Jack's sister Susie and her best friend Binkie make occasional appearances in the books; they hate the Secret Seven and delight in playing tricks designed to humiliate them, although this is partly fuelled by their almost obsessive desire to belong to the society.

The names Secret Seven and Famous Five had already been used by the author Charles Hamilton, under the pen-name Frank Richards, in his long-running series of stories featuring Billy Bunter and Greyfriars School.

The Secret Seven was the name of a secret society that featured in a series of eleven stories published in The Magnet magazine in 1934;[citation needed] the term "Famous Five" dates from 1910 and is applied to a group of the leading characters: Harry Wharton, Frank Nugent, Bob Cherry, Johnny Bull and Hurree Jamset Ram Singh.

[citation needed] After corresponding with the real-life Peter, in 1948 Blyton published her first Secret Seven story, which describes how her fictional society came to be formed.

It followed an earlier short story, "At Seaside Cottage", which introduced the leading characters, Peter and Janet, prior to the formation of the society.

These were left uncollected until 1997, when all but "At Seaside Cottage" were published in a single volume by Hodder Children's Books under the title of Secret Seven: Short Story Collection.