Matches is not, actually, there to keep an eye on Eel for Batman. He's on a case, infiltrating a new gang in town--the same thing Matches Malone has been doing ever since his first death.
Eel's there for the same reason. Investigating, clearly--pose as a criminal, break up the gang as a hero, same thing he's been doing for he hardly remembers *how* many years. Both Eel and Plas live in the moment, one job at a time, one joke at a time, and if the rest of the world is not the same as it was--well, he still has Woozy Winks to come home to, as constant as his own rubber flesh; and there's always plenty of crime for Eel to commit and Plas to fight.
Neither Eel nor Plas was, especially, a detective. When the suggestion was made, these days, Plas would exclaim, indignantly, that he was being confused with the *other* stretchy guy; and of course Eel was just doing his dishonest day's work, never dream of betraying his buddies in crime, serious.
Nevertheless, certain patterns did become rather obvious, and Matches Malone was one of them. He was a *nice* pattern, sure--in a back-alley never-speak-of-it-again sort of way that reminded Eel rather of the good old days, when he was young and not yet two people. Eel liked Gotham, even if Plas didn't, altogether--it seemed *older* than most other cities, grimy in a grand old corrupt manner that was only enhanced, in the last ten years or so, by its new protector (who was hardly as powerful as the old Green Lantern had been, anyway, and with a much less exciting costume).
And Matches was Gotham, in a way that predated the Batman and the his flashy sidekicks and rogues, and which made Eel O'Brien ache with nostalgia. If he reaches his mind back, he can almost remember *knowing* Matches in those days, if not before Eel was Plas, at least to the time when "crime in Gotham" meant gangsters and small men for hire like Eel himself.
It makes it easier than usual to hold his shape when pressed, as is becoming an increasingly regular event, between one of Gotham's grimier alley walls and Matches' body--complete with match, and on those occasions when Eel had a moment to think, between the sensation of Matches within him and the strain of not losing his shape under the thrusts, he was quite impressed that Matches could hold on to it under the circumstances. Perhaps he just wasn't *that* good a back-alley lay--but to be more than mediocre at it, he would be more than Eel, and if he were more than Eel, he would be Plas, who did not, as a general rule, allow himself to be fucked by common criminals.
(He had Eel for that, after all. And Woozy hadn't been a criminal in years, not since Plas had just been Eel, so he didn't count, especially since they never did it or mentioned it in public.)
It was the common criminals' loss, anyway. Plas was a *much* better lay than Eel, if he did say so himself--much more flexible, if nothing else, handsomer *and* with a better sense of humor. Confident, too, and didn't all the books say that was what you needed?
Besides, he had evidence. The only time the match even *started* to slip behind his neck was when Eel's clothes slipped to expose a flash of red, his joints bent despite his will in a *slightly* more than merely human angle, or--oddly enough--when he couldn't resist making a joke halfway through.
It was really too bad Matches couldn't meet Plas, instead, but then Plas would have to arrest him and Matches wouldn't be Matches anyway, not without his match.