What qualities or traits do you look for in a co-founder?

In the early stages of making a connection with a potential co-founder I have a set list that I check over. I also think this can vary across the type of business you are trying to launch as some businesses for example could require a co-founder to be based in the same city/ locality for it to be able to operate efficiently and successfully. A digital agency like you've mentioned you're looking to start up would not necessarily need to have co-founder locality as a prerequisite however there may be other unique factors to your industry that will be a must in a co-founder that others may leave off their list.

With that said, I do have a general check list when vetting co-founders:

1. Complimentary strengths - For me it's understanding my weaknesses and the parts of the business I would struggle to tackle myself. In my case I would say I don't have a strong design/ branding skillset so that is something that has always been a crucial criteria point for a finding a co-founder. I have thought about outsourcing this part of the business however in my case I think it is important to have a founder on board that would own this side of the business as I feel it would better carry the brand long term.
2. Resilience - There are bound to be many difficult and often de-motivating periods in the early stages of a start-up. It's how you bounce back from those that defines you as a founder. Having someone to be able to bounce ideas off of when times get difficult and the right change is needed can be a lifesaver. This can be hard to gauge when you may not know the other potential co-founder too well but sometimes maybe just asking the right situational questions can help surface how they might react to such a situation.
3. Integrity - I've heard some nightmare stories from other founders about co-founders going rogue and completely losing sight of any ethics involved in running a company. This can obviously cause huge amounts of damage to your brand and business so best to find someone with their integrity fully intact.
4. Common values - Adding on to the previous point, I would say just general common values are important in working in a cohesive way with a co-founder. These could be factors such as providing a supportive and flexible working environment for employees or engaging in practices that best protect the environment.
5. A genuine shared interest in the vision - Another one that's high up on my list. I do believe that without a genuine interest in a company's products, services or brand you will not end up putting 100% into it. Shared passion makes working cohesively so much easier and as good as someone may be in their expertise, without passion the creative juices that can carry a companies growth just don't fire the same.
6. Availability - Might come across as an obvious one for other people but identifying just how much time the other person can realistically commit to the start-up is important. If they work a full-time job and have time after work and perhaps a day or two on the weekend to commit then that's great. If they don't have that time and i'm expecting 20-30 hours a week from them then it sets unrealistic expectations from the start which will lay a poor foundation. I think this is a two-way street in the sense that both sides need to be up front and honest about how much time realistically they can commit!
7. Energy and drive The final quality I try to gauge when vetting a co-founder is their general energy and drive. I need someone who will have enough gas in the tank to put in the extra hours required to get an idea off the ground.

That's my core criteria, as mentioned anything else depends on the type of business!