The world of Naruto is filled with its fair share of strange abilities and character designs. There are ninja who can turn into water, have mouths on their hands or use the bones in their body as weapons. Most of the time, this strangeness is explained as a bloodline limit that is passed down through the family or as a forbidden jutsu that was performed on the person. The exception to this is the Akatsuki member, Kisame Hoshigaki.
Kisame was one of the Seven Swordsmen of the Mist and one of the earliest members of the Akatuski. Compared to his peers in both groups, Kisame looks very different. The swordsman has blue skin, beady fish-like eyes, sharp triangular teeth, and what appear to be multiple sets of gills all over his body. Kisame clearly looks inhumane and strikingly fish-like, but why is that? Why does Kisame look so different from everyone else in the world of Naruto?
Maybe it Runs in the Family
Although what gives Kisame his shark-like appearance has never been directly stated in Naruto, there are a few possible answers. It could be that Kisame possesses a bloodline limit that alters his physical appearance. During one of the anime-only arcs in Boruto, a character named Shizuma Hoshigaki was introduced. This character was a relative of Kisame and had the same shark features.
The Hoshigaki clan may carry a bloodline limit that makes its members look more shark-like. This would make sense, as the Hidden Mist Village, where Kisame is from, had several clans with bloodline limits that were purged. With such a prominent bloodline limit, the Hoshigaki clan would have been quickly targeted. This purge may have left Kisame and Shizuma as some of the last surviving members of this clan.
If this theory is true, the fact that Kisame's entire clan was wiped out could have been another factor that pushed him to abandon the village. Living in a village whose citizens slaughtered each other needlessly might have added to his disillusionment with his home. However, there isn't much hard evidence for this. The Hoshigaki clan is only mentioned in this one anime-only arc of Boruto, so it's hard to say how much of it is fully canon.
Maybe It's a Tradition for the Seven Swordsmen of the Mist
One of the few features that Kisame shares with other people is his teeth. Almost every member of the Seven Swordsmen of the Mist has the same sharp, triangular teeth that Kisame does. Zabuza, Suigestu and Chujuro all possess menacing-looking teeth. It seems to be a tradition among members of the group and possible future members to reshape their teeth into this more fish-like appearance. This tradition could have been started for a number of reasons. For example, it may be an intimidation tactic to scare enemies or an homage to the sharks that surround their water-locked village. Whatever the reason, it's clear that the swordsmen are intentionally making themselves appear more shark-like.
The prospective users of the sword Samehada, known as Shark Skin, could take this tradition one step further. Maybe those that know they will inherit Samehada alter their bodies to look even more shark-like. The person who wielded Samehada before Kisame was Fuguki Suikazan, who was not a member of the Hoshigaki clan. Nonetheless, Fuguki had the same beady fish eyes as Kisame. Either through jutsu or surgery, those who know they will be Samehada's next master could be changing their bodies to be just as shark-like as their sword.
Maybe Kisame is Just an Old Design
It has been noted by the author of the Naruto manga Masashi Kishimoto that his original idea for the Akatsuki was a group of monsters that lacked human features. Some of that original idea can still be seen in many members of the Akatsuki who have inhuman features. Deidara had mouths on his hands, Sasori's entire body was a puppet and Kakuzu's body was stitched together with multiple hearts. Kisame fits this original monstrous idea as well. He is a towering shark person who would easily fit the description of a monster. When Kisame fuses with Samehada, he becomes even more monstrous, growing a tail and fins that make him look like a sea monster from a horror movie.
There may be no in-universe reason why Kisame has shark features. The true reason may ultimately just be that he's a carry-over from an old idea. Whatever it may be, Kisame's design does its job well, making him one of the most iconic members of both the Akatsuki and the Seven Swordsmen of the Mist.