Yay! After the ant arc I finally have something worthwhile to say about their similarities. They both use fire! Feitan's "Rising Sun" ability and Hiei's dragon both employ fire! Besides this, they're both really fast and rather cold in personality.
However, Feitan's nen type is transformation...and I'm guessing that Hiei's is probably manipulation (well...he does manipulate the dragon, and doesn't just transform it out of air...), so their nen types aren't really the same. (although, who am I to judge on Hiei's nen type?)
Besides this, don't they look really similar here...the hair...the eyes...(well Feitan's are a bit narrower than Hiei's...) They also both seem to be on the shorter side.
Kuroro and Treize
These two are quite...interesting. No, they don't look alike by any means, but they certainly do act similar. Both have the charisma, the leadership skills...Treize was in charge of OZ, which is techically the 'enemy' side in Gundam Wing, similar to how Kuroro was also on the bad side of HxH.
From these two, two other similar characters in Gundam Wing and Hunter x Hunter pop up into my mind...^^:
Pakunoda and Lady Une
Okay, so their main similarities are that both were unexpectedly nice and very loyal to their leader (Kuroro/Treize). Well, I guess Lady Une had sort of a mean side and a nice side, while Pakunoda didn't have a split personality...
Not really the hair--Hiei's is more spiked up and it's black, but minus the hair, Killua and Hiei have the same face--similar eyes..etc.
Kuroro and Creed
Their personalities aren't anywhere the same, but their faces sure look similar:
