A perpetually relaxed, easy going character that rarely gets frustrated or excited over anything.
Ever noticed that guy that never really gets mad or excited, or really any emotion outside nonchalent. Meet the Mellow Fellow. Easy going and satisfied with their position to the point of near stoic. Often a benevolent or at least affable character due to rarely being confrontational. What deferenciates this from his more serious cousin The Stoic is that while the latter rarely conveys real emotions outside blunt seriousness, the Mellow Yellow is usually more along the lines of being very positive and relaxed in nature, to the point it nulls out any other emotion. Usually this trait has the side effect of making them seem rather slow or dim. In most cases however, such a character can be incredibly perceptive or philosophical. At the very least, they often have a hidden tint of intelligence beneath there. Very often their inability to take offense gets exploited and makes them an Extreme Doormat. They are often very flexible for the use as The Chew Toy, due to their ability to suffer endless abuse without expressing much harrowing pain to the ordeal. Don't think one is incapable of biting back however. After all you can still be assertive without losing your cool.
Examples Anime and Manga