Mystery Data (ミステリーデータ?) are objects from the Mega Man Battle Network series that can contain Battle Chips, Zenny, objects specific to an area, or can be viruses in disguise. Mystery Data can be found in colors of green, blue, purple, and gold and are usually found in the corners of Internet areas. In the first Mega Man Battle Network, there is little to no distinguishing features between the data colors, and thus almost all forms of Mystery Data behave like green Mystery Data.
Green mystery data can be found in nearly every area of the main areas of the net. Green mystery data is unique from any other color in that they reappear every time MegaMan jacks out of the net. These can contain Battle Chips, amounts of zennies, BugFrags (Mega Man Battle Network 4 and later) or disguised viruses. Some Battle Chips can only be obtained from green mystery data (without use of the Chip Trader). Using the UnTrap Sub Chip will defuse trapped mystery data, revealing rare data that would not appear in other green mystery data.
Green mystery data contents are randomly chosen from a small selection of possible rewards. In Mega Man Battle Network 2, the reward is confirmed once the player picks up the data, and thus the player can repeatedly reload a save file until he gets the desired data. This has changed in future games, where the content is randomly decided the moment MegaMan jacks in and will not change if the player reloads a save file after the data is picked up.
In Mega Man Battle Network 4 onwards, green mystery data can also appear during battles against a virus. If MegaMan can defeat his foe(s) without letting the data get destroyed by any attacks (including his own), he will be rewarded with its contents in addition to his normal results. While any attack, including the Mega Buster, can shatter battle mystery data, chips like the Gun del Sol and Django series won't harm it, even if the mystery data is within the ray. If MegaMan obtains the data from a green mystery data that appears during a boss battle, the rewards will be substantially larger - usually a rare chip that cannot be obtained anywhere else. Also, while the chance of encountering mystery data in battle is low, it has a somewhat higher chance of being found when MegaMan ventures into areas that are either off-the-beaten path or side routes. While it still can be found on the main paths, as it's randomly encountered, it can be found somewhat easier when MegaMan is not on a main path.
Blue mystery data is usually found at least once in all areas of the net, like green mystery data. Unlike green mystery data, it can only be acquired once in a preset location and does not reappear once gathered. Its contents are also preset and do not change over multiple playthroughs.
The exception to this rule is Mega Man Battle Network 4, where acquired blue mystery data reappears once the game begins again on a higher difficult level with new items. The contents of blue mystery data are also greater, usually containing rarer Battle Chips, more Zenny, Power Up Items or even Navi Customizer Programs.
In the cyberworlds of enemy computers, blue mystery data may contain keys or other objects needed to unlock doors.
Purple mystery data is a rare type of data that first appears in Mega Man Battle Network 2 alongside the Sub Chip system. Like blue mystery data, its contents and location do not change save for different difficulty settings in the fourth game. It also requires an Unlocker Sub Chip to open. However, its contents are great, resulting in very rare Chips or programs.
Gold Mystery Data
Gold mystery data is the rarest of all mystery data. It only appears once in a great while in place of green mystery data that appears around the net. They can be opened like green mystery data and usually contain large quantities of zenny, Bug Frags or a rare Battle Chip (including Mega Chips).