So if you look at Star Trek - which really minor character has had the most appearance or impact on the secondary literature/games/comics/other? So not a Kyle or a Leslie or a Mbenga but someone along the level of Mr. Adventure or even more blink and you miss them than that?
Robin Lefler in the New Frontier novels. Was a minor TNG guest, turned into a main litverse charakter.
See for me she's too memorable - I'm think more people who are just there to advance the plot - in Star Wars terms think the guy there to hold the cushion with the medals on it.
Kell Perim, Insurrection. Showed up in the novels just to disappear again. The death of the actress makes her too obvious, too. I would think.
Shelby too, also NF. Sonya Gomez became a main character in the Starfleet Corps of Engineers series of ebooks. Is she minor enough? The masked delegates in The Undiscovered Country had a whole novel about them (they were Romulans in disguise!)
I've gotten two novels and three e-novellas out of Lucsly and Dulmur of the Department of Temporal Investigations. Several others have written about them too.
I'd say Isaac Newton. Really has only one or two brief appearances (depening on if you count the TNG holodeck character appearance as "real" or not, the other in VOY), but has been referenced to in literally thousands of articles (and a huge impact to boot). Some of it even outside Trek!
Two notable picks: -The Bajoran Sisko tells "you don't belong here, go home" in Rapture becomes a Vedek in the novels. https://memory-beta.fandom.com/wiki/Yevir_Linjarin -The Prime Universe version of a Mirror Universe Bajoran guard Bashir fights and steals the phaser from which he uses to kill Mirror Odo in Crossover becomes the first officer of DS9 in the novels. Prime Universe Mirror Universe IIRC, in the novels she resigned from Starfleet in protest of the Tezwa fiasco.
Lori Ciana has racked up an impressive number of appearances in Trek Lit considering that she only appears as an unnamed victim of a transporter accident in TMP.
I only write for my own enjoyment, but I had Leffler, Gomez and Shelby together in one story about the development of an anti-borg weapon.
But Melora gets a whole episode to herself with Julian, so is she strictly spear-carrier minor as JoeZhang outlined? I looked up DS9's Sakonna and was not surprised to see she'd turned up in couple of novels