Territorial, ill natured, bossy, aggressive, taunting and animated. They will swoop at animals and people alike. I have watched a crow chase a hawk and then a mockingbird will chase them both. There is really nothing outstanding about their appearance, monotone gray and white with the occasional brown patch.
What is stand out about the mockingbird is their song. Not a single song but made up of a melody of other bird songs. A mockingbird can have 250+ songs in their repertoire. Their melodies seem to adjust with the time of year and what other birds are dominant in the area. Back in the spring after the arrival of the Great Crested Flycatcher to the yard who was being very vocal, the yard "mocker" picked up sounds very similar.
Their song may consist of up to 15 songs sung in a continuous string. So despite their dull appearance and less than desirable behavior at times, their song adds a pleasant melody to the yard.