When work began on Mega Man X, Keiji Inafune, the father of the series, originally envisioned it with Zero. Plans changed and X became the hero instead, but Inafune had his cake and ate it too once Zero became something of a fan favorite. He would become a staple of Mega Man X shortly after, with multiple subplot focusing on the character and of course: the ability to play as him.
After Mega Man X kind of got away from Inafune, he had his chance to give his true creation a starring role with the Zero series. Inafune would continue to work at Capcom, having a hand in Mega Man ZX and ZX Advent, spiritual successors to the Zero series, and Star Force: before unceremoniously departing in 2010.
Nearly everything in that timeline is worth revisiting: including all of the entries in the Zero/ZX Legacy Collection.
Source: Destructoid Review: Mega Man Zero/ZX Legacy Collection