Zeno Zoldyck (ゼノ＝ゾルディック Zeno Zorudikku) is Killua's grandfather and Silva's father. A witty old man, his advancing age has not dampened his quick thinking and speed in battle. Zeno wears alternating signs on his garb that say either "A Kill A Day" or "Never Retire." He appears to be on good terms with the Hunter Committee's Chairman, Netero, who hired Zeno to assault Ming Jol-ik's palace in East Gorteau in order to catch the chimera ant King off guard. Zeno favors Killua more than his other grandchildren, most likely because Killua is the heir of the family.

Source: HunterXHunter.wikia