Note: Requested and inspired by Salia Glacerious' comment, "How is kisame a shark like person?" and a recently released episode of Naruto.
Warnings: None
Pairings: None
Character: Kisame
As a newborn, Kisame didn't look all that different from other healthy babies of his generation. Admittedly, his hair did stick up more than the next infant's and his teeth were a little pointier than usual, but that was hardly abnormal enough to warrant extra medical examinations, especially in the Village Hidden in the Mist, where such features weren't all that uncommon.
The blue skin tone, gill-shaped markings, and round, fish-like eyes by which he would later be recognized and feared were acquired through something more "special" than mere genetics.
It began when Kisame killed one of the previous Seven Swordsmen, and inherited the demonic, sentient sword known as "Sharkskin," or Samehada. It was common opinion among the Seven Swordsmen, and indeed, the Mist Village as a whole, that Samehada was an especially sinister tool of war, due to its love of eating the blood and chakra of its wielder's opponents (which it did regularly and of its own volition). The cute little skull-shaped nub at the end of Samehada's handle didn't help it seem especially benign, either.
Samehada's habit of sucking up an enemy's source of energy was a useful feature in and of itself, but what made the sword all the more desirable was how it fed the extra chakra back into its wielder, thereby increasing his fighting power and stamina.
But there was a rather interesting side effect to absorbing and using this foreign chakra, which Kisame soon discovered as he began using Samehada.
The chakra that had been devoured by Samehada and then spat back into Kisame took on the qualities of the sharkskin sword, and therefore it began to influence some of his physical features.
This phenomenon of foreign chakra molding the appearance of a ninja was not unheard of; Jinchuuriki commonly showed characteristics of the beast they hosted. Indeed, the One-Tails' hosts were known to have dark coloring around their eyes, reminiscent of a raccoon, while hosts of the Kyuubi might have the whisker markings of a fox on their cheeks.
Likewise, as Kisame continued to utilize the chakra provided by Samehada, his teeth became far sharper and pointier than was their natural right to be, and gill-shaped slits appeared on his cheekbones.
However, it was only after the first time he merged completely with his sword that he started looking decidedly similar to a shark. In a desperate battle with an unexpectedly powerful opponent from the Land of Earth, Kisame merged his body with the flesh of his sword, thereby increasing his power by tenfold and winning the fight. However, some of the the acutely sharklike physical characteristics that he'd gained from the stunt persisted even after he broke the merging jutsu with Samehada and wrapped the sword back up in some fresh bandages.
He didn't particularly care that his skin had turned a permanent shade of blue and that his eyes became round and beady.
His newfound appearance scared the living daylights out of his opponents, and that was fine by him.
Note: This one's more of a stretch then some of the other stories, since Kisame already looked rather sharkish before he had Samehada, but I still think this explanation makes more sense than the one I would've used if I wanted to stay completely canon. (Which had to do with coming from a clan with sea-creature features, for those of you who want to know... Introducing Kisame's long-lost sister: Kiuni! Aren't her sea-urchin-spine-hairs and intestinal mouth just so appealing?)
If you review, I'm sure Kiuni will be kind enough to restrain her kisses... :P Just saying.