Co-founder

Choosing who you start with, the earliest and hardest-to-undo decision.

Also known as: co-founder, cofounder, founding team

A co-founder shares ownership, risk and decisions from the start — unlike an early employee, who trades work for salary plus a smaller slice. The right co-founder covers your gaps, argues with you productively, and stays when it gets hard. The wrong one is the most expensive mistake a startup can make, because unwinding it costs equity, morale and sometimes the company.

good split:  you build × they sell (or you + domain expert, or you + operator)
bad split:   two identical engineers, best friends, no uncomfortable conversation yet

Choose for complementary skill, shared commitment and conflict compatibility — in that order. Friendship is not a qualification, and neither is brilliance without reliability. Work together on something real before incorporating: a weekend project reveals more than ten dinners.

The classic mistakes:

Equal split by default, no vesting. 50/50 on day one with no vesting means a founder who leaves in month three keeps half the company. Agree the split deliberately and vest everything.
Nothing in writing. Handshake deals between friends feel safe until they are not. Sign a co-founder agreement while you still like each other.
Adding co-founders late for small contributions. Someone who “helped with the logo” is not a co-founder. Late additions get advisor equity or a job, not a third of the company.
Ignoring the breakup conversation. Ask now: who decides when you disagree? What happens if someone leaves? If you cannot discuss it, that is the answer (see co-founder conflict).

If you go solo instead: see solo founder. Either way, decide the equity, vesting and exits before the company is worth arguing about.