Playing through any of the game modes, including online matches with optional goal objectives, will net the player experience points that may be used to unlock alternate costumes, music, concept art, and other rewards. The campaign of "Injustice: Gods Among Us" was written by NetherRealm Studios in collaboration with DC Comics' writers. Described by NetherRealm as "Story Mode 3.0", the "Injustice" campaign was approached similarly to "Mortal Kombat vs. DC Universe" and the 2011 "Mortal Kombat" reboot through the use of a cinematic narrative versus the traditional ladder-based single-player experience. According to lead designer John Edwards, the plot is meant to rationalize the game's fighting mechanics between characters that would not normally fight one another and explain how Batman can "stand toe-to-toe" with Superman. Writers Justin Gray and Jimmy Palmiotti served as story consultants to ensure that the comic characters kept their proper voices in "Injustice". The roster was selected internally by NetherRealm Studios with input from DC Comics. The main goal during the selection process, which took around three to four months, was to create a cast featuring a wide variety of characters. Characters were chosen based on criteria such as size, ability, gender, popularity, and how well they fit within the "Injustice" storyline. The addition of each playable hero and villain required about six weeks of development. The characters were designed with the intent to make each fighter unique, and fighting styles and movesets were tailored to the lore and personality of each character. Ed Boon stated that DC Comics had given NetherRealm a large amount of freedom with their properties, allowing them to put their own spin on characters. Several other characters from the DC Universe, such as Darkseid, Scarecrow, Metallo and Gorilla Grodd, also appear as non-player characters, making cameo appearances in story cutscenes, stages and other game modes. Outside of the DC universe, Scorpion from the "Mortal Kombat" series appears as a downloadable guest character. The playable characters are: Additionally, several characters are playable as alternate skins for existing characters. They include:  Lastly, some characters and alternate skins are exclusive to the iOS and Android mobile app.  : Available as downloadable content; included in the "Ultimate Edition". In an alternate universe, the Joker tricks Superman into killing his pregnant wife Lois Lane, which detonates a nuclear weapon that destroys Metropolis and kills millions. Enraged, Superman kills the Joker and over time murders more criminals, causing him to grow more corrupt and ultimately become a ruthless dictator.