It's not unusual to see anime characters transform into animals or super-powered versions of themselves, but it's not every day that you see anime characters who can turn into weapons. This unique ability can take many forms, including everything from turning a single finger into a gun to fully transforming into a katana.
The most well-known example of this kind of transformation is Soul Eater Evans and the other Demon Scythes attending Death Weapon Meister Academy in the 2008 anime Soul Eater. These characters work together with shinigami in training to become full-fledged Death Scythes. Another beloved example is Yukine from Noragami. Yukine is Yato's "shiki" - meaning, that when the god Yato calls him by the name he gave him, he transforms into a powerful sword. Not all of these transforming anime characters come from anime that take things seriously, however - there's also a series of 1-minute shorts called Henkei Shoujo which features girls who transform into everything from fighter jets to mecha.
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When most people think about anime characters who transform into weapons, they think of Soul Eater - both the anime itself and the character who it's named for, Soul Eater Evans. Soul Eater is a Demon Scythe, which means that he's a human-weapon hybrid who can transform into a scythe-like weapon at will. His goal is to become a Death Scythe - the key weapon wielded by Shinigami. To do this, he'll have to defeat 99 evil entities and one witch. With the help of would-be Shinigami Maka, he takes on as many enemies as he needs to in order to achieve his goals.
There are other characters in Soul Eater who transform too - worth mentioning are Tsubaki, who can transform into a wide variety of weapons, and Patti and Liz, who work with the head Shinigami's son.
In the world of Noragami, gods can create contracts with the spirits of those who died prematurely. The spirits are turned into Shiki, which are bound to the gods and given a name. When that name is called they automatically transform into a magical sword. Yukine is a Shiki who has partnered with Yato, a minor god who is trying to make a name for himself. The path to forging a good relationship is a rocky one - Yukine passed away at a young age and still has the mindset of a middle schooler, and he justifiably resents Yato for making him into a Shiki and then failing to scrounge up enough money to feed and house him, but the two eventually begin to get along.
Yukine transforms into a katana and eventually a dual weapon once his bond with Yato becomes strong enough. He mostly uses him to fight supernatural monsters, but occasionally the duo is also forced to fight other gods.
Though not especially known for this ability, Naruto does occasionally use a transformation jutsu to take the form of a shuriken (ninja throwing star). He does this so that allies can use him as a projectile or disguise himself if he wants to hide from his opponent.
After Shinichi Izumi's right hand is taken over by a parasite named Migi, he begins to experience some major physical changes - including the ability to transform parts of his body into ax-like blades. While this ability is exclusively under Migi's control, it's still Shinichi's hands that it's happening to. Typically, he has to use his ax hands when fighting other people whose bodies have been taken over by parasites.