The Interpreter Alien is a little yellow alien with six arms and the only one that could apparently translate The Great One's "language". He doesn't appear to know how to fight and he always appears to be excited over every little thing, such as when Ben was proclaimed the 'new leader' of the trio of aliens (the Leader Alien and the Enforcer Alien), the Interpreter alien started jumping up and down and cheering. Like Leader Alien he can also shoot laser beams from a tech device in his forehead. He also seems to treat Ben more fairly unlike the others.
If you look closely, when Ben and Gwen were being carried off by the aliens in Ben 10: Secret of the Omnitrix, one of the aliens that appear was similar in species and was shown with an eyepatch.
If you look closely in The Purge, you can see one of the alien prisoners has same species as the Interpreter Alien, but in a different color. And in Nor Iron Bars a Cage, it's the same alien as in The Purge.
The three aliens are similar to the three branches of government: the legislative that makes the law (Leader Alien), the judicial that interprets the law (Interpreter Alien), the executive that enforces the law (Enforcer Alien).