Ever heard the expression, "Still waters run deep"? That's basically how we feel about Colonel Brandon. On the outside, he's a quiet guy – at 35, he's a bachelor approaching middle age, and he doesn't seem to have any family to speak of. He's dignified and well-to-do, but beyond that, we don't get much from first impressions. The Colonel appears to be rather dull to the insensitive eye, but beneath his tranquil surface, there's a whole lot going on. Elinor is the first to appreciate this, but gradually, other characters (including, finally, Marianne, when she decides to marry him) realize that he's a whole lot more than meets the eye.
First of all, Colonel Brandon is clearly a sensitive soul, even though his exterior seems unflappable and even rather remote. He's the only person who appreciates Marianne's music the same way she does – respectfully and thoughtfully – and we get the impression that his quiet persona hides a deeply intellectual inner self. Colonel Brandon is also by far the character with the most significant emotional trauma to deal with, which he manages to do in an admirably mature, applause-worthy fashion. He's the ultimate combination of feeling and logic, and comes off as the only real grownup in this whole cast of characters.
All in all, Colonel Brandon may seem to be on the boring side, but he's actually not – he's just more under control than the other folks we meet here. We don't get to know him too well, but we can imagine that he's a pretty rewarding friend to have, once you get past his rather stiff exterior.