In genre fiction no one goes it alone. Or at least very few. This is one of the top three observations I've made as a reader over the years. As in life, everyone should have someone. This applies in romance and it applies in genre fiction as the main character runs through the pages of the book, saving the world, and surviving the harrowing circumstance that writers conjure up on the way to completion.
Let's look at a few of the authors I like in the genre fiction world. I tend to pick adventure, thriller, suspense. Clive Cussler, Robert B. Parker, Robert Crais, and Harlan Coben are at the top of my list, although many other authors also have some of my money as well. Cussler brought to life the fabulous pairs of Dirk Pitt and Al Giordino (the first pair), Kurt Austin and Joe Zavala, as well as the rare married pair of main characters Remi and Sam Fargo. Robert B. Parker brought Spencer and Hawk to the reading world and I thank him for it. I've passed many late nights with those two. Robert Crais brought about Elvis and Pike. Another pair well worth the money and the time to read and reread. Last but not least is Harlan Coben. His fertile noggin produced Myron Bolitar and Win, whose last name I cannot remember. Possibly because in the books he is always just Win, the nasty badass partner that comes in and cleans up as needed.
Not all of my favorites come in paris though, so there are exceptions. Other favorite authors include Brad Thor, Ted Bell, Ben Coes, and the sadly departed Vince Flynn. Scot Harvath, Alex Hawke, Dewey Andreas, and Mitch Rapp, the characters belonging to those authors do not generally have tight partners appearing throughout the book. If you compare Spencer and Hawk to Rapp you see an immediate difference. Likewise with Elvis and Pike compared to Scot Harvath. Same drill.
Interestingly, at least to me, there could be a third group in this discussion. James Rollins writes another bunch of really awesome books in the same categories. While I've read all of his books and greatly enjoyed them, his characters, to my mind, are in a group. Not so much an MC and a sidekick. He features a main character that is at least part Native American. He also features the rare pair of partner characters who are married. Quite the ground breaker this Rollins guy. Well worth reading! As an aside, one of his books has the most compelling interesting and fascinating look at quantum mechanics that I've ever come across. Moreover, it was in a religious/prayer context. Incredibly well done.
So of course it can work in many ways ways, but pairs of characters enter my writing thoughts first. Especially Spencer and Hawk from Robert B. Parker and Elvis and Pike from Robert Crais. Dirk Pitt and Al Giordino follow closely along with Kurt Austin and Joe Zavala, all four of those characters coming from the fertile mind of Clive Cussler.
In all cases, the bad guy always seems to be going it solo. That's probably another topic though.
So while your MC is out saving the world, consider writing in a partner watching the shadows and providing the comic relief and additional drama that only a partner can provide!