As you can see from these comments by me, @seijitsu, and @eha1234, there's a considerable amount of skepticism over how "recycled" these designs really are.
Me: Personally, I don't think the characters in this question even
look that similar, not compared to Ken Akamatsu's Naru and Asuna, or
the Ai Yazawa and Naoko Takeuchi characters that seijitsu mentioned in
regard to the "same faces" question.
eha1234: Yusuke and Gon have very different personalities and I think
the green scheme is only present in the anime - the manga is black and
white. Hiei and Killua's demeanour isn't the same. Killua, when
relaxed, is quite playful whilst Hiei is silent. They've both got
tragic pasts though. Many of Togashi's characters have androgynous
appearances, especially in Hunter x Hunter: Kalluto Zoldyck and Pitou
for example. I would say although the character designs are similar in
some respects, they're all unique and different too.
seijitsu: Yuusuke does arguably wear a green seifuku in the manga, but
otherwise I agree with eha1234 that the characters' personalities are
not very similar and with Torisuda that the characters do not look
If Yusuke and Gon both had hair which was a particular shade of periwinkle only expressible in the CMYK color space, I might agree that the similarity was significant, but black hair doesn't quite make it. You can see characters which are much more similar in appearance in seijitsu's answer to "Why do anime and manga use the same faces?". And if having multiple characters with androgynous appearances constitutes recycling, then all shoujo artists have been recycling the same male character design at least since the 1990s. If anything, Togashi is doing better than many mangaka at not recycling character designs, as eha1234 points out.
There are answers to the question "Why would a mangaka recycle the characters' visual appearances?" over at "Is it common for mangaka to maintain the same character design between different titles?", so I'll focus on the personality issue. At this point we're really talking about writing, which cuts across all media. If you follow the works of individual writers of any medium—literature, manga, Western graphic novels, visual novels—you'll notice that it's quite common for writers to have characters with similar personalities across several works. For example:
- Stephen King often has a tough, brassy guy who swears a lot that everyone looked up to; he also likes to have a kid or intellectually disabled man with psychic powers.
- Gen Urobuchi likes to have an idealistic character whose ideals are a little too rigid, and slowly destroy everything the character believed in before sending them to a miserable death (Sayaka in Madoka and Kariya Matou in Fate/Zero). I asked a question about whether the similarities between another pair of Urobuchi characters was intentional.
- Ken Akamatsu had three manga starring an energetic, caring, somewhat tsundere girl with thick eyebrows (Cindy in AI ga Tomaranai, Naru in Love Hina, Asuna in Negima). Two of these manga (AI and Love Hina) also had a young blonde troublemaker, and two of them (Love Hina and Negima) had a shy and quiet girl with short hair and a secret, passionate crush on the main character.
There are lots of reasons why writers do this. Sometimes it's because of a theme that cuts across all of their work. Sometimes the character represents some portion of the author's personality that keeps seeping into their work. Sometimes the author wants to put similar characters into very different situations and see how this changes things. I haven't read or watched Hunter x Hunter, but from what I understand it's more lighthearted than Yu Yu Hakusho; perhaps Togashi wanted to explore how similar characters would behave in a more lighthearted story. From what eha1234 says in the comment quoted above, there are also some quite profound differences in personality between the Yu Yu Hakusho characters and the Hunter x Hunter characters; it's possible the purported similarities are a complete coincidence. We can't know exactly unless Togashi stated in an interview somewhere why he did it, or whether he even intended to do it.
Of course, in a visual medium like manga, if the writer reuses a certain personality, it often makes sense to give that character a similar appearance as well, because appearance and personality are closely related in anime and manga.