One could argue that when the series began, he was not only the real star of the series, but also one of the single most useful companions on the physical scale, often knowing how to assemble basic tools to solve problems and not at all a bad fighter. Ian was often the most vocal about challenging the Doctor's plans, and as a result, he clashed with the First Doctor more than any of his other companions. This is in contrast to the more understanding Barbara, who generally didn't challenge the Doctor as much, but only because she didn't always see a need to, and would often challenge everybody equally on serious moral matters, one thinks of The Aztecs.
Ian was often one of the Doctor's snider companions, and the ultimate Captain Obvious, as though he was an intelligent person who seldom had intelligent things to say, frequently making immediately observable observations rather than coming up with many new observations of his own. That was often Barbara's job, or the Doctor's. Despite this, he was also the most fiercely loyal when a decision was made and he accepted it, even if he often expressed concern over potential consequences of the whims of Barbara and the Doctor.
The actor who played the character, William Russel, has returned to this character every so often for specials, as can be seen here:
