No one would choose to spend their life marooned on an island,
(well, maybe for a few weeks - if there was room service)
most of us need human contact. We need people to interact with, to bounce ideas off of, to laugh with and to learn from. Everyone needs a friend.
Certainly, your main character needs one.
Your protagonist is busy pushing your plot ahead
(or busy drinking at the bar - I'm not going to judge)
and they need someone at their side. Think carefully about this someone. They can be more colourful than your protagonist; after all they're not the one in the starring role. Quirks are fun. Perhaps they are the owner of a large slobbering dog who has a habit of leaving puddles of saliva in unfortunate places.
(I've just named the dog and have a picture of the beast in my head. This is sad.)
They can't be smarter than your protagonist but nothing is stopping them from having insights your main character is unable to grasp. They can be more observant or more successful. They could have a happy marriage while your protagonist can't keep a relationship afloat for longer than a week, or the reverse might be the case.
How long have they known each other? Since school? Since childhood? Or is this a working partnership, decreed by powers above and both of them have to adjust and figure out what works? The length of the relationship will determine the level of trust between the two. Trust is earned - it isn't transmitted through that first handshake of introduction.
(What is one of them has a wart on their hand? I digress...)
This character can say things your main character never could. They can have rougher (or smoother) edges. Your protagonist can get themselves into dicier situations because they know they've got someone to call to get them out - or at least someone to come looking for them.
Give your protagonist a buddy. It's a lonely world without one.
By the way...here's the dog.
His name is Alphonse.