one-dimensional

one-dimensional (comparative more one-dimensional, superlative most one-dimensional)

1. Having length, but no width, height or depth.

   A straight line is one-dimensional.

2. (figuratively) Lacking depth or believability; flat.
   • 1972, Hardie Albright, Stage direction in transition

     Such one-dimensional characters have always existed in the theatre along with more recognizably human characters.

   • 1999, Kimberly Kimbell-Lopez, Connecting with traditional literature

     In contrast to this are the flat, one-dimensional characters found in fables.

3. (figuratively) Lacking personal depth and substance; conceited, vain, shallow, superficial (see also; two-dimensional).