It's good to have people on your side. While it's probably too bold to claim that you should never start a company alone, it's absolutely true that having other founders alongside you will make the journey easier and give you valuable insight.
Because much of the value of co-founders is being able to arrive at decisions via debate and controversy, the most important criteria (besides, perhaps, being invested in the project and being a problem-solver) is that you are comfortable disagreeing with your co-founder. For this reason, it can be risky to bring people onto a project because you like them as people. Opt instead for people whose drive, intellect, and opinion you respect, but who you're not afraid to disagree with.
<aside> 🏇 Having a capable team can't be understated. Much of what potential investors will look at is whether they believe you and the other founders have the chops too execute on your idea. Be willing to set aside the politics of your personal life and only bring on co-founders that you believe will be real assets to the project.
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Short answer: unevenly.
Even if it's just a 51% - 49% split, it is likely to be important at some point down the line. Being able to tie-break disputes is too vital to sacrifice it to avoid an uncomfy conversation early on.
Valid question, because I had never heard the phrase before taking my entrepreneurship course. Essentially, company founders often perform poorly as executives once the company has grown past the startup stage$^2$.
An important realization for many entrepreneurs is that the set of skills used to start your company successfully will not be the same set of skills necessary to run your company successfully. At that point, it will be necessary to abdicate leadership to somebody with the right skillset, or to grow your own. This is one of many areas where your mistakes will likely become a swift teacher.
Before you can grow a company out of the startup stage, though, you're going to need to provision it.