Kazuki Muto (武藤カズキ, Mutō Kazuki) is a 16-year-old boy who attends a private academy with his sister. After becoming involved in Tokiko Tsumura's fight and subsequently being killed and revived by a Kakugane. He is told to return to his ordinary life, but instead insists upon assisting Tokiko in her battles. Kazuki is very enthusiastic friendly and caring for his friends and sister, Mahiro; He tends to put himself in a difficult situation in order to assist or protect others. Although he has an unusual way of showing it, he does have feelings for Tokiko Tsumura. Many people note that Kazuki can be a hypocrite; the first instance was trying to stop Koushaku Chouno from becoming a homunculus and becoming, albeit against his will, a Victor due to his Kakugane. This is pointed out many times by Koushaku.

The Kakugane Tokiko used to revive Kazuki turned out to be Black Kakugane III disguised as a normal Kakugane, which transformed him into a Victor. He is then pursued by the 'Muto-assassination team', as well as his former commander, Captain Bravo. During his assassination, he was dubbed 'Victor III' by the Alchemy Warriors. Afterwards he was given his freedom as he had promised to be killed if he can not find a cure for the Victor transformation before two weeks; the amount of time it takes the Black Kaugane to permanently change him into his Victor form. In the end, he sacrifices himself when he sends himself and Victor to the moon with Sunlight Heart +. In Buso Renkin Period, however, he returns to Earth successfully with help from Victor's Buso Renkin, Fatal Attraction. As soon as he returned to Earth, he and Papillon settled their scores, with Kazuki claiming victory. After this, he returns to a normal life with Tokiko.

His Buso Renkin is Sunlight Heart (+) (サンライト・ハート , Sanraito Hāto). It first it was believed to be Kakugane LXX), Kazuki came to discover he was actually in possession of the Black Kakugane III; with the actual Kakugane LXX being in the possession of Dr. Butterfly. Seeing as the Kakugane doubles as his heart, his Buso Renkin plays a unique and critical role in Kazuki's survival. Once invoked, his Buso Renkin, Sunlight Heart, takes the form of a spear-lance with a large decorative cloth attached. He was told his personality fit as a spear so his Kakugane took that form. The cloth is turned into an energy force when it resonates with Kazuki's willpower. The cloth's energy can be used as propulsive kinetic energy for forward charging, or, dissipating the gravitational forces of a fall. This ability is the origin of its name as the light it radiates shines like the rays of the sun.

As the story progresses, Sunlight Heart changes into Sunlight Heart +, with a smaller, sleeker design. This new streamlined version, at first glance, seems inadequate as a lance due to its decreased reach and size. However, Sunlight Heart +'s true properties reveal themselves as being able to vastly increase its reach and power; responding to Kazuki's willpower to unleash its self-contained energy, disassembling into, and arranging its core components in the most efficient formations. To this extent, it activates its alchemic powers in the same manner as the original Sunlight Heart. Though lacking the cloth, it can generate propulsive forces from the point at the end of the handle (which is shaped like the original Sunlight Heart, though much smaller) which opens to release kinetic energy. It can also fire off blasts of internal energy.

Sunlight Heart + is last seen in "Buso Renkin Period," when it was destroyed by Papillon's Buso Renkin, Neardeath Happiness, with only the original spear's head remaining; however since it is seen that Buso Renkin regenerate as time passes, Sunlight Heart would've been repaired. Unlike most of the other Kakugane; which were collected and hidden away when the Alchemist Warriors suspended operations, Kazuki was allowed to keep his since he cannot survive without it.Voiced by: Jun Fukuyama (Japanese), Steve Staley (English)

Source: Wikipedia