The main four characters of these series are practically twins because of their characteristics. Yu Yu Hakusho and Hunter x Hunter are both created by Yoshihiro Togashi in which Togashi even based most of these characters from Hunter x Hunter off of his past series, Yu Yu Hakusho.
Yusuke Urameshi → Gon Freecss
Kuwabara Kazuma → Leorio Paladiknight
Kurama → Kurapika
Hiei → Killua Zoldyck