Strangely enough in at least the first three films in the very loose series is that Drummond's sidekick Algy is played by the same actor - Claud Allister! And he even shows up as another character in 1938's ARREST BULLDOG DRUMMOND. When you first encounter him in BULLDOG DRUMMOND (1929) you assume that he's going to be someone left behind as the action begins because he is the epitome of an upper class twit. Indeed, I suspect he could have competed very strongly in the Upper Class Twit of the Year Contest!
But as that first movie goes on he proves himself too be - although perhaps a little behind the always quick-witted Drummond - an eminently capable person willing to jump into the fray and do the right thing. It helps that in playing this role Allister is so clearly having a grand time mixing things up with both his fists and his rather agile eyebrows. His eyes go from haughtily clenching a monocle in perfect imitation the dumbest silver spoon buffoon you've ever seen in your life to directing menacing glares at the villains he's helping Drummond take down.
It doesn't matter if it's Ronald Coleman, Ralph Richardson or John Lodge in the title role as long as Claud Allister is in there giving it his all, you've got something to look forward to seeing.