The early days of comic books ('38+) saw numerous titles containing a wide assortment of characters in each issue. Harvey Comics, created by Alfred Harvey in 1941, followed that line but gave each participant 10-12 pages to tell the tale rather than a more common 4-6 pages. Since adventure and action were the purpose of the stories, this added space is filled with derring-dos and rowdy fisticuffs rather than character development so we do not learn that awfully much about British Agent 99 but it certainly gave 99 a lot of room to do a bunch of different things in each of the books.
Alas for whatever reason, 99 did not last past 4. Neither did the comic book. Pocket Comics, the magazine that presented the Douglas adventures, came into existence in August of 1941 and stayed for only 4 issues before leaving.
One interesting aspect of Pocket Comics was that instead of being 'normal' comicbook size, it came out as a digest which the publisher promoted as being pocket-sized, hence the name.
Issue 1 had the characters of Satan (wow!), Red Blazer, Spirit of '76, the Black Cat, the Phantom Sphinx, British Agent 99, and the Zebra, each with 12 pages, as well as Spin Hawkins with 4 pages.
Issue 2 saw Spirit of '76 battle Satan in 24 pages followed by Red Blazer, the Zebra, the Black Cat, The Phantom Sphinx, and British Agent 99 12-pages adventures, a Spin Hawkins 4-page story, and a Private Pinky 2-page. It also had a text short story about the the Black Cat.
In Issue 3 there was an 11 (maybe 12) page story about Spirit of '76, 12-page stories each about the Zebra, Red Blazer, the Black Cat, 10 pages of adventures each for , The Phantom Sphinx, and British Agent 99, Spin Hawkins with 7 pages, Private Pinky with 2. Also there were two 2-pages text short stories, one with the Zebra and the other about the Black Cat.
Finally, Issue 4 gave 12-page stories each for the Black Cat, Spirit of '76, and Red Blazer and 10-page stories for , British Agent 99 (joined by the Black Cat), The Phantom Sphinx, and the Zebra. Private Pinky had a 2-pager, a character named Simpy had a single page, and Spin Hawkins had an 8-page adventure. There were also two text short stories, one with someone called Captain Freedom and another with the Black Cat working with Spirit of '76.
Most of these characters would disappear like British Agent 99 when the comic came to an end. One, the Black Cat, would live on for a very long time in other comics as well as her own.