 In the cooperative campaign , a separate story involves two robotic characters and GLaDOS . The designers initially planned to use Chell and a new human character called " Mel " . GLaDOS ' dialogue would play off the humans ' " image issues " , and this aspect was retained after the designers switched to using robots . GLaDOS seems troubled by the robots ' cooperation , and tries to aggravate their relationship through psychological tactics , such as praising one robot over the other . Valve initially considered having GLaDOS deliver separate lines to each player , but they found this to be a significant effort for minimal benefit . The writers also tried adding lines for GLaDOS that would encourage the players to compete against each other for rewards , such as meaningless points , but playtesters did not respond well . Faliszek said that in cooperative games , it can be difficult to deliver key dialogue or in @-@ game events to players , who may not be looking in the right direction at the right time . Instead , using their experience from previous games , Faliszek and Wolpaw kept the story and key comedic lines short , and repeated them frequently . 
