The Engineer was a companion of the Doctor. He was there, with him, from the beginning, having actually been friends with Omega. Despite his title, he was actually a bred military man, but still a brilliant engineer. During the time of the Fourth Doctor, he had a romance with Romana's companian of his species, having previously had a romance with Sarah Jane Smith. During the events of the Five Doctors, he found himself nearly dead, having only survived due to interferance from his old friend, Rassilon. When infected with the fungus a few weeks later, he regenerated. During the Time War, he regenerated 13 more times. He is currently the last of his species, having killed the Master's friend of his species. During the events of Stolen Earth, his TARDIS was shot down by the Daleks, forcing him to regenerate, though it was noted that his TARDIS was simply damaged and not destroyed, repairing itself and then coming to the rescue of the Doctor and the companions.
Although all of his personalities have been different, they have shared common traits. He is brave, carefree yet responsible, and intelligent. He is mostly peaceful, but he has been known to use weaponry when deemed necessary.