In all honesty I think that all that is needed is a dose of reality.
If this to-be-hired person is going to be on-par with the existing "founders", in terms of pay, equity, and ability to vote to affect the outcome of the company, then I would call them a co-founder.
If instead this to-be-hired person is going to be a very senior employee with more equity than later employees, and some input on the product management stuff, then they are an employee, not a co-founder.
I do not know which case is true, my only point is that we do not need to invent new pseudo-titles for everyone to feel special.
If this to-be-hired person is going to be on-par with the existing "founders", in terms of pay, equity, and ability to vote to affect the outcome of the company, then I would call them a co-founder.
If instead this to-be-hired person is going to be a very senior employee with more equity than later employees, and some input on the product management stuff, then they are an employee, not a co-founder.
I do not know which case is true, my only point is that we do not need to invent new pseudo-titles for everyone to feel special.