Perhaps first on everyone's list for the second tier of characters from Detective Comics next to the Batman Family (excluding long-underwear types like Crimson Avenger, Air Wave, etc) would be these two who appeared right from the start.
"Slam" Bradley and "Speed" Saunders... two early recipients of nicknames in comics for which they'd be forever attached. And they could not be more different in personality, while sharing many other common elements.
Slam was witty and gruff, Speed was serious and polished. While Bradley was a private detective attached to Cleveland's police department however loosely, Saunders was evidently a federal agent attached to the harbor patrol. Nevertheless, each adopted other identities and/or disguises to get up close and personal with the criminal element they wished to dethrone.
While Slam had a full-time loyal, albeit slightly incompetent, counterpart in Shorty Morgan, Speed typically worked along...his potential sea-side sidekick Red Pennington then occupied with the likes of Lt Commander Don Kerry of Anchors Away. Decades later, Bradley would work alongside fellow detectives of the same publication as Roy Raymond, Human Target and others cementing him on Earth-One. For Saunders, it was revealed later on that he was first cousin to Sheila Sanders (different spellings among their folks) aka the Hawkgirl of Earth-Two, with Speed remaining in retirement to let a new generation take up the call to action.