In some novels, the main character can stand alone. This works in stories where the conflict is internal to the main character, or novels where it's the main character against everyone else.
However, for most story lines, you might want to consider adding a partner or sidekick to support the main character. What's the difference between a partner and a sidekick?
Some of the difference is in how seriously you take the secondary character. Does the secondary character have a detailed backstory and take up almost as many pages as your main character? If you answered 'yes', then your main character has a partner for his/her adventures.
If your secondary character receives far less time on the page than the main character, and only show up to crack jokes or drive the getaway car, then he or she is probably a sidekick. There's nothing wrong with that, but it's important to know the difference.
In my current novel project "Paradise Found", the main character (Shane Carrigan) has an unwelcome and uninvited partner, none other than the Archangel Michael. There's no way I could write Michael the Archangel as a humorous or forgettable sidekick, so he's definitely Carrigan's partner. (That's a lucky turn of events, since the case Carrigan is working involves trying to avert the Apocalypse...)
In the writing prompt below, take the secondary character I've described, and write about him two different ways. Is he the main character's sidekick, or equal partner?
Writing Prompt: As I surveyed what was left of my racing car, I couldn't help but feel lucky to have a grease monkey like Lou on my pit team. He had a way of taking what looked like scrap metal and somehow transforming it back into a car that not only ran... it would help me leave my competitors far behind. Lou was...
Have fun, and keep writing!