In the series Mobile Suit Gundam Wing, Zechs Marquise (ゼクス・マーキス, Zekusu Mākisu?, taken from German sechs, lit. "six"), is a legendary Mobile Suit pilot, belonging to the OZ Faction. Renowned for his amazing skills in combat, his exploits earned him the title of the Lightning Count (Lightning Baron in the original Japanese dialog). A personal friend of OZ leader Treize Khushrenada, Zechs is often the commander of important missions, and receives his orders directly from Treize.

However, Zechs has a very important past. His real name is actually Milliardo Peacecraft (French 1,000,000,000 - milliard, one thousand million in long scale, one billion in short scale, continuing the series' named for numbers scheme.), heir to the former kingdom of peace, the Sanc Kingdom. After the destruction of his homeland by United Earth Sphere Alliance forces, he donned his iconic mask, and left behind the name of Peacecraft, for his new moniker, Zechs Marquise. He is also the older brother of the pacifist heroine, Relena Peacecraft, and the rival of Heero Yuy throughout the show.

Source: Wikipedia