Not all eumonetic energy is alike. It comes in different types, corresponding to the four elements: Fire, earth, water, and air. These four elements form a circular scale, with the elements being separated by 90 degree angles. Fire and water are at opposite ends of the scale, as are earth and air (see above).
All eumonetic energy is elementally charged such that it lies somewhere on this ring. Think of this sort of like electric charge. Where electricity has positive and negative charges, eumonetic energy has fire, earth, water, and air charges. If you combine a positive electric charge with a negative electric charge, they cancel out. Similarily, if you combine a fire charge with a water charge, they will cancel out. On the other hand, if you combine a fire charge with a wind charge, you will get a charge that lies at a 45-degree angle between them, and you will lose some energy in the process. (In more scientific terms, we would refer to stored eumonetic energy as "eumonetic charge", not energy.)
As mentioned previously, every living being has its own internal eumonetic energy. In fact, it is this very energy which allows life to exist. Any object which is not eumonetically charged cannot possibly be alive or have thoughts. Of course, since all eumonetic energy is elementally charged, this means that all living beings have an elemental "affinity", which is defined by their position on the ring.
A being's affinity is determined by many different factors. Affinities are not constant, and can change over a being's lifetime. In fact, almost every aspect of a creature's life has an effect on its affinity. Every action that a being performs, every object they touch, and every event that occurs around them has an elemental nature, and will tend to push that being's affinity one way or the other. The effects are usually very slight, but over time they can make a difference.
For example, someone who spends a lot of time swimming will probably tend towards a water affinity. This is a very literal example: since the person is around water a lot, their affinity tends towards water. On the other hand, someone who spends a lot of time fighting will tend towards fire. This one may not seem so obvious, since no actual flames are involved, but the act of fighting is a fiery act.
Even every-day acts like eating and breathing have an effect. In general, however, they tend to balance out: eating pushes you towards earth, breathing pushes you towards air, drinking pushes you towards water, and exercising pushes you towards fire.
A person's affinity tends to determine their personality. Fire affinitives tend to be hot-headed, fierce, controlling, active types. Water affinitives are kind, gentle, caring, and laid back. Earth affinitives are dilligent, hard-working types who are slow to start but hard to stop once they set their mind to something. Air affinitives are quick, witty, impulsive types. (See this thread for some more ideas on elemental classifications.)
It is important to note that elemental affinity does NOT distinguish intelligence or alignment. A person anywhere on the ring could be a genius or an idiot. Furthermore, fire affinitives are NOT necessarily evil, and water affinitives are NOT necessarily good. (There really is no such thing as "good" or "evil" anyway. It's all relative. But that's beside the point.)
Elemental affinity is, of course, a huge factor in the abilities of any user of eumonetic techniques. Naturally, every technique has an elemental bias, falling somewhere on the ring. A person using eumonetics will be most efficient when using techniques that exactly match their affinity. When using techniques that don't quite match, some energy will go to waste. Specifically, the fraction of the energy that is wasted is equal to one minus the cosine of the angle between the elemental charge of the energy being used and the elemental position of the technique. In other words, if the technique is more than 90 degrees around the ring from the user's affinity, that user simply won't be able to use that technique. A water affinitive can't use a fire technique, etc.