Alien X is the Omnitrix's DNA sample of a Celestialsapien from the Forge of Creation.
Alien X has three personalities: Serena, the 'voice of love and compassion', Bellicus, the 'voice of rage and aggression' and Ben, the 'voice of reason'. All three of these voices combine to form Alien X's speech. Otherwise, Alien X is silent. The only things he will say are what he will be doing - what Ben, Bellicus and Serena have agreed to do.