Killua's yo-yos

Killua is in possession of two specifically designed Yo-Yos. He uses them as his weapons of choice. The yo-yos are extremely durable and heavy, each weighing 50 kilograms, due to their being made out of a special metal alloy, which is also an excellent electrical conductor. It is unknown how he came to own them, although it is likely he acquired them while he was out of Greed Island to pass the Hunter Exam.

Appearance
In the manga the yo-yos have a six point star or hexagram design on them. The hexagram design is used in many various religions and cultures including Islam, Christianity, Occultism, Hinduism and especially Judaism where the hexagram is the symbol used for "The Star of David". In the Greed Island OVAs the Hexagram or Six Point Star is changed into a Pentagram or a five point star. However, later in the Chimera Ant arc, Killua is seen using a yo-yo that has a pentagram on it. Since Killua has two yo-yos, it is most likely that one yo-yo has a hexagram while the other has a pentagram.

Greed Island Arc
Killua first shows the two yo-yos to Biscuit as part of his preparations for the battle against the Bomb Devils, announcing his will to combine them with his Hatsu. Being the first time he uses them in combat, he experiments during his fight with Sub, defeating him.

Chimera Ant Arc
Killua uses one of his yo-yos to grab Ikalgo before the latter falls in the underground lake, where his blood-frenzied Chimera Ant subordinates would tear him apart. During the infiltration into the palace, he obliterates the heads of two insect soldiers with them to save Ikalgo's cover, although this fact leads to Welfin growing suspicious of Flutter. He later attacks Palm with one of his yo-yos, only for it to be crushed by the reborn woman.