Zero's Role in the Megaman Series
Zero- not much was known about him until Megaman X4 and Megaman: The Power Battle 2. In Megaman X, he was just a mysterious red robot that worked with X and had that unusually powerful Z-Buster which he removed in favor of keeping his powerful but short-ranged Z-Saber. Now for then, Zero's role was Megaman's best friend in the new future where he lived a new life as X. In Megaman X1, he sacrificed himself to save Megaman from Vile. In Megaman X2, his body parts were being hunted by Serges (possibly a robot version of Dr. Wily) who was later revived in the process. In Megaman X3, he was partly playable with a cheesy Z-Buster and if at full charge, he releases a slash beam which made him cheesy but he was merely a back-up. Now here's when Zero really gets truly playable...
Yup that's right in Megaman X4 but he lost that Z-Buster which for me has defined his personality. While somewhat harder to use, but Zero learns different sword skills along the way and two, that saber doesn't require charging, it's pretty strong itself despite its short range. In X4, Zero gets his own mode different from Megaman. Most of his weapons earned take form of a saber skill which most of them run on unlimited energy. Also, he's a pretty fun player when you get to know him. What we also learn in X4 is his mysterious origin- he was Dr. Wily's final robot built to destroy Megaman X but he instead after some time, became a force of good. He was pretty feral to succeed his supposed "father" Dr. Wily plus he had the evil energy from Megaman 8 (which was 100 years prior to this game). I thought he was pretty serious and tragic, plus he didn't even realized he saved the world even if he can't save Iris. But he was not used by Dr. Wily since he was still unstable but he was probably hoping that this robot would succeed him. Sigma reveals to Zero the past.
In Megaman X5, we learn Sigma was going to use him to bring him back to his original design. Also in X5, gameplay has changed where you can switch between Megaman and Zero when tackling stages. If you choose to play him in the beginning, he has his rather weakened Z-Buster which while it packs a punch, still can be fired only when standing up. Here, after the Enigma fails, Zero volunteers to pilot the Shuttle to change the Colony's crash course. If the second mission fails or two, time runs out Zero goes Maverick which can lead to a bad (but rather hilarious ending). If he succeeds, Zero will be remembered by Megaman X which is canon because it goes to the next game where he is found.
In Megaman X6, Zero must first be fought before he can be used yet again. Compared to X5, X6 Zero has a stronger Z-Buster shot but still can't be charged and needs to be fired while standing and now has a new kind of sword. In his role here, Zero's DNA was being used by Gate which again adds conflict to the story. Zero's ending is weird though that he seals himself again which the time and place when it happens is unknown.
By X7 to X8, Zero loses his Z-Buster yet again for some reason and it's back to sabers. In X8, he has a variety of short ranged weapons though he had one bladed staff and two, he can later gain the Sigma Saber which is a very awesome weapon. In Megaman X Command Mission, Zero is back with Megaman with him using different type of swords.