4 Must-Have Qualities of a Reliable Startup Co-founder A mirror image of yourself won't work.

These days, more startups are going the route of launching with multiple co-founders and building traction on the side before eliminating the team's full-time income. While bringing on a co-founder can increase your likelihood of success, there's no denying that a bad co-founder can quickly sink any side project before it has time to flourish.

So how do you ensure that you pick a reliable cofounder? I had the opportunity to sit down and interview co-founders Jen Kessler and Chiara McPhee of the email marketing platform Bizzy (recently acquired by SendGrid), as well as investor Kat Mañalac of Y Combinator, who recognized their potential and helped them through the accelerator program.

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During our interviews, it became clear that a few key qualities can make all the difference in determining whether someone will truly be a reliable co-founder. So which attributes matter most? Let's take a look.

1. Skills complimentary to yours.

One thing that quickly became apparent in both interviews was the importance of complementary skill sets. Mañalac explained, "You want to work with someone who isn't a clone of yourself. ...Maybe one person is more technical and more product-focused, and the other person is more focused on sales and talking to customers. Ideally you'll have complimentary skills."

Kessler further expounded on this when she noted, "When you are a small team with limited resources, you need to make sure that everybody brings something new to the table and has distinct roles and responsibilities. The art and science yin-yang has been a big asset in [my] time working [with McPhee]."

Simply put, your company isn't going to last very long if you and your co-founder are both programming experts but have zero marketing experience. Your founding team needs to have a broad range of skills to successfully manage the many demands of running your own startup.

2. Niche-specific experience.

An additional area to consider with complimentary skill sets is the importance of at least one of the founders bringing niche-specific experience to help guide the company's development. As Mañalac noted, this can make a strong impression on investors, because, "when founders are building something to solve a problem they've experienced, they're much more tightly tied to that problem and they understand the problems and user base more intuitively."

This is a common trait of successful founding teams. My friend Jon Read of Keet Health, a communication platform for medical providers, highlights a real-life example of how niche experience made a difference for his company. "Ripe from my own experience as a patient, I had a basic idea of how I wanted to improve doctor/patient communication, but I didn't understand how things worked from the medical provider's perspective. My co-founder, David Self, had the medical experience and expertise necessary to give us a 360-degree picture, and this completely changed our trajectory. Without his input, we would have missed the mark entirely."

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3. Level-headed with their emotions.

Even if you're completely enamored with your co-founder at first, it's important to remember that the honeymoon phase isn't going to last forever. As Mañalac explained, "All co-founders will fight at some point. There's no escaping it. The question is, how are you going to respond to each other when you do come to a point where you disagree on something?"

It's essential that you choose to work with someone who remains calm and level-headed, even in stressful situations where you might disagree with one another. Someone who can't control their emotions could have destructive results for your startup. Because of this, Mañalac suggests working with potential co-founders on a trial or freelance basis at first so you can better gauge their decision-making ability.

As David Weston of Informed Education explains, failure to remain level-headed during stressful situations can have serious consequences. He writes, "we are less likely to think things through calmly and logically. Our decision-making is more haphazard and more likely to be sub-optimal. We are less likely to take a team perspective in groups, leading to lower performance as a team."

4. An equal share of the greater equity pie.

When joining a startup as an additional co-founder, knowing where you stand as far as equity is concerned will make a big difference for the long-term health of your relationship. As Mañalac noted, "Equity conversations are something that create a lot of tension later on. Sometimes you get a founder who came up with the idea and then six months later the other co-founder joined and you'll see an 80/20 equity split. The truth is the vast majority of your company's life is in the future, so you should be on as equal a plane as possible."

If a founder isn't willing to provide a more equal split of equity, you're most likely going to be better off searching for a different startup opportunity. You shouldn't give up 20 percent of your future equity just because you came onboard a few months later.

Related: How to Find Great Co-Founders for Your Startup -- Fast

Choose wisely.

Co-founder selection will ultimately prove to be one of the most important business decisions you ever make. Don't fall into the trap of deciding to work with someone just because they're a friend or family member.

When you choose to work with someone who makes up for your weaknesses, is equally committed to your startup's success and will treat you with fairness and respect, you'll be well on your way to achieving your business goals.