The horns on a Ralts' head allow them to sense emotions in sentient life; they share happiness and sorrow with their trainers, are frightened by anger, and sway gently when they sense nervousness, all of these reactions growing in intensity with those of the feelings they detect. Needless to say, this ability to sense the nature and strength of human emotions have made Ralts a remarkably useful diagnostic tool since the development of psychiatry. In more recent years, and in some ancient and forgotten ones, Ralts were used in a different science, for detecting the existence of emotions can be as valuable as detecting their power.
Whether as attempts to build a true AI, to create godless killing machines, or as accidental byproducts of technology, humans have at times created new species of pokemon. Whenever they did, it was a Ralts which detected the moment these new creatures gained sentience. For some, like Porygon's creator Bill, this was a joyous occasion, or at least it was one until his Ralts began to share the intense stress that shortened the first Porygon's life. For others, such as Giovanni of Team Rocket, Ralts' reaction was a disturbing sign, for Mewtwo was never intended to be a thinking creature, merely a living weapon. And in ancient Unova, Ralts' reaction was an incredible surprise, for the people who had designed Golurk as mobile suits had never even imagined they would take on a spirit of their own, and only realized the sounds they made were the language of pokemon when their Ralts reacted to their heroic dreams with a powerful aura of their own.
A Ralts' expressions are simply the reflection of the emotions of others; what they truly feel is forever unseen. Perhaps they are the loneliest pokemon of all.