IANAL, but I would tend to agree with your co-founder. First, you openly admit that you are using the "Yo" brand name to drive adoption to your app: "having yo

IANAL, but I would tend to agree with your co-founder. First, you openly admit that you are using the "Yo" brand name to drive adoption to your app: "having yo in the name is crucial for our app to gain traction". Second, you market it as a "more complete version" of the Yo app. Third, just because you are in a different country than Yo, doesn't mean that they still can't/won't try to sue you. They just got a fresh round of funding, obviously have bigger plans for the brand, and as such will probably want to squash out someone infringing on their brand quickly, especially one that is marketing and branding to solely based on their brand. Just bringing the lawsuit will cost you money that you probably don't have and will easily put you in debt without the case even getting to a court room. Fourth, the name "Yo Messenger" makes it sound like a different product under the Yo brand thus causing easy confusion with consumers and I am sure any court would probably see similar. By your logic if back when Facebook was just getting started if I had launched an app called "Facebook Messenger" then that would not be infringing on the Facebook brand.