A sales practice customer and a personal conversation partner have different jobs. Before choosing the voice or appearance, I define what the character is there to do and what a good interaction should leave the user with.
For a practice customer, that might mean keeping an objection in play until the salesperson addresses it. A conversation partner needs a different rhythm and a clear decision about what to remember. The personality has to make sense in the product where it lives.
I write that into a persona profile: role, manner of speaking, behavior, memory and boundaries. Then I check it against situations that ask something of the character, including a change of topic or a repeated question.
The AI Personas demo shows a fictional character with a visible profile. Try the conversation, look at the role behind it and describe the character you would want in your own product.