Haney also had 'pet' heroes he loved to (over)use as co-stars in the '70s: Sgt, Rock, Deadman(both in the same story, along with Sherlock Holmes...none of whom actually meet Batman, but instead save him at long-distance); Wildcat(just as Rock survived WW II in Haney's mind, no matter what Bob Kanigher said, Haney likewise ignored 'parallel earths' where Wildcat was concerned). He also featured Green Arrow and Black Canary plenty(although they were only together in # 100, with GL and Robin). He likewise avoided some heroes like the plague; Batgirl, despite all those years as the co-feature in 'Detective Comics', only showed up once(sharing a guest-shot with Wonder Woman, who appeared only slightly more often), while Hawkman and Flash had very long gaps between their first and second meetings with Batman(Hawkman's second teaming actually trumpeted 'For the first time in 10 years!' on the cover.
Haney never featured Supergirl, who showed up in 1978 in an issue Haney didn't write, and again just over a year later, shortly after he leftt he book for good. Zatanna only turned up once, after Haney's tenure. Interestingly, one last Sgt. Rock team-up finally used the WW II Earth-2 setting, and in the last year of the book, Robert Kanigher himself wrote issues starring his own creations, Rose and Thorn, and Ragman, three characters Haney probably never heard of!

Haney always said he didn't care what characters were used - B & B co-stars were all based on sales.

(He also refused to write a Batman/Supergirl team-up, though. So grain of salt time.)

A case can be made that Doctor Doom became the pet character of every creator from Lee/Kirby to Byrne. Doom made to look bad? Doombot! I have to admit that I like the idea since I'm a huge fan of the character.

Generic Howard Chaykin character-comes in many names from many publishers but its generally the same guy

Danny Chase was another Wolfman pet. Marv seemed convinced that he was the greatest character in Titans history. Every one else (especially fans) found him a whiny annoying brat.

The Golden Age/Earth 2/'the one in the JSA' Hawkman(what IS the politically correct term these days?) is a possibility for Roy's 'pet' character. Thomas often wrote about Carter Hall in glowing terms, and because that character appeared in every single JSA story in 'All-Star Comics', Thomas went out of his way to include at least a single-panel 'disembodied ghost head for reference' appearance in every issue of 'All-Star Squadron'.
Late in the run, Thomas actually forgot to put in a photostat image of Hawkman's head in a final-panel shot, along with other JSAers who had not actually been in the story, but were being referred to as 'waiting for the rest of us' at headquarters.
So, Thomas actually reprinted the panel in the next issue's letter column, with an image of Hawkman added where Thomas had intended it to be. Thus he 'retconned' an error out of existence!
EDIT: Late last night, I thought of two even more obvious examples: In WHAT IF # 13, the issue in which Conan visits 1977 New York City, he meets a cabdriver called 'Danette', after RT's then-wife. That name later pops up in 'All-Star Squadron', when the hero known as Firebrand (Rod Reilly) is wounded at Pearl Harbor and replaced by his sister, also named Danette.
Neither character is exactly a 'Mary Sue', though Conan's cabbie friend is sort of a 'muse'. Firebrand-Danette, however, does not really dominate the action in the Squadron's title(if anything, Liberty Belle, along with husband-to-be Johnny Quick, have the mosttime in the spotlight).