Your companions are a vampire spawn who finally achieved his freedom after 100 years only to realize that he might be turning into another monster, a priestess of an EVIL goddess who wants to blanket the world in eternal darkness, a man who sold his soul to a devil so he could play hero, a mage who seeks out ancient artefacts to absorb their magic so he can stave off some curse, and a gythyanki warrior who is being enslaved by her races ancient enemy and has to work with those she deems beneth her.
Also, Larian studios also stated that this was going to be an "evil" game.......not everything can be sunshine and rainbows.
Actually, Larian just said that they included the evil companions first and that they will add the good companions later. Sunshine and rainbows are on their way.
