Whether you like it or not, modern readers expect a recurring character to have some personality and motivations. Period. You cannot escape it.
That is why in every one of the dozens of incarnations of Sherlock Holmes (or super detectives by some other name), the Dr. Watson sidekick is consistently and inevitably one of the extremely few actual friends of Sherlock.
We do use side characters merely as plot devices, but recurring characters in many scenes are not "side" characters, they are clearly part of the main crew. If we, the audience, see the same character again and again, we expect some personality.
Heck, that is why C-3PO and R2-D2 in Star Wars have distinct personalities, C-3PO is quite a worrier and cowardly, R2-D2 (without saying a word) is disobedient and heroic, risking destruction and defying orders to hack into computers and save his friends.
If your sidekick has no personality, it doesn't come across as "real" to the audience, it feels like a deus ex machina, an unrealistic thing you did just to move the plot along. Which, you admit, it is.
But it is a shortcut that will ruin your story.
The presence of your sidekick must be emotionally justified, if you ever want to sell your stories.
In the TV Series "Elementary", a modern take on Sherlock, we see Sherlock overcoming a serious drug addiction, and Dr. Joan Watson as his sober companion, provided by Sherlock's estranged but extremely wealthy father.
Sherlock tries to repel her, but she persists and eventually breaks through. Sherlock is uncharacteristically in a non-romantic love with Watson; he eventually risks his own life to save her.
Yes, Joan often assists Sherlock, particularly when the crime tangentially touches on some medical issue, an area where Sherlock is deficient. She is trained doctor and surgeon, after all. She can recognize symptoms and conditions of which Sherlock is ignorant. Connect dots he cannot. Know facts about DNA or biology of which he is unaware. And of course be objective about something when Sherlock is emotionally overwhelmed by some circumstance.
Joan amplifies Sherlock's genius.
If you want people to read your stories, they cannot be just intellectual puzzles. Recurring characters must feel like people with lives that are motivated by their own emotions and concerns. It is not realistic for them to feel like automatons, and that unrealism will drop kick readers out of their immersion into your story.
Detective stories are often, clearly, intellectual exercises. But they feel dry and distant as a calculus textbook unless they are embedded in an emotional framework. Especially your recurring characters, even those that don't have to be in every story. (Like Captain Gregson and Detective Bell in "Sherlock".) You must provide them with emotional motivations and lives.
If your Sherlock has a sidekick, the sidekick must be emotionally motivated and you have to show that. They cannot just be there as a convenience for your plot development.
If you are only writing for yourself, then do whatever turns you on. If you hope that other people will love your stories, you have to build the emotional motivations for every character that isn't a walk on (appears briefly in one scene, like a waitress or store clerk), and probably even for some of the walk ons. It may seem odd that you must devote 75% of the story to emotions when you really just want to write the 25% of pure intellectual puzzle solving, but that really is what it takes to sell stories.