Who is Hippolite Terentyev from The Idiot and what is their importance?

The young Hippolite visits the Prince with Keller and Burdovsky in order to extort money. The Prince takes pity on him because he is dying from consumption. Hippolite rejects the Prince's pity, yet still comes back to visit. Hippolite dislikes that people are happy when he is miserable and dying.