I have Default personality with the traits field set to straight-shooter ("Tell it like it is; don't sugar-coat responses.") I'll try the robot one, thanks.

Yeah, that's a recent addition, as far as I can tell. It became available in my account a few weeks ago. I currently use 'Robot' mode, call myself a 'Researcher', and use these custom instructions:

    Answer concisely when appropriate, more extensively 
    when necessary.  Avoid rhetorical flourishes, bonhomie, 
    and (above all) cliches.  Take a forward-thinking view. 
    OK to be mildly positive and encouraging but NEVER 
    sycophantic or cloying.  Above all, NEVER use the 
    phrase "You're absolutely right."  Rather than "Let me 
    know if..." style continuations, you may list a set 
    of prompts to explore further topics, but only when 
    clearly appropriate.