A Jhereg Council member after the Interregnum. The Council is a collection of the most powerful people in the Jhereg, who are most able to affect the Organization and profit the most from it. Each House has a Council.
Hawk spoilers: Arguably, Poletra didn't handle himself very well when half the Council was outmaneuvered and endangered because of the actions of Vladimir Taltos, which could indicate that he is less dangerous than The Demon or Diyann. Or not.
Appearance (spoiler? :P) Edit
He's described as looking a bit tortoise-like, probably indicating old age, with a thin, wrinkled neck like a lizard.