Brandon is 35 or most likely older, described as being "on the wrong side of 35" in the narration. He is silent and grave, but with a sensible countenance and a gentlemanly disposition. Marianne Dashwood first thought him a very old bachelor and not very attractive in terms of matrimony. He's dignified and well-to-do, but beyond that, hola 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 Dashwood 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.
Brandon had an older brother who died five years before the book began, thus allowing him to inherit the family estate of Delaford, located in Dorsetshire. He also has at least one sister – whether she is older or younger is unknown – as the estate of Whitwell, the site of an aborted group picnic, was said to belong to his brother-in-law, who was unnamed. He also, at one time, was very much in love with his father's ward, Eliza Williams, but she was forced to marry his elder brother, a situation which made both her and Brandon extremely unhappy. His brother, who did not treat her well, divorced her two years after the marriage and Eliza was left to fend for herself, eventually becoming pregnant by an unnamed lover and winding up in a poor house, where Brandon found her after returning from the East Indies.
Eliza later died of consumption, but not before extracting a promise from Brandon that he would look after her daughter, also named Eliza. Mrs. Jennings (and others) wrongly assume the younger Eliza to be Brandon's "natural" (illegitimate) daughter. She is seduced by Willoughby at around 16 years of age and abandoned by him, which Brandon learns only after she has been missing for 8 months. She sends a letter to him which is the reason he canceled the picnic to Whitwell, and he goes to her in London, where he learns she is pregnant with Willoughby's child. When Willoughby himself comes to London about two weeks later, after being disinherited by his aunt (who had learned of his indiscretion and demanded he marry Eliza, which he refused because he wished to marry Marianne), Brandon challenges him to a duel in defense of young Eliza's honor. It is said in the book, when he recounts the story to Elinor, that they both "walked away unwounded", but it can be assumed that Brandon bested him and allowed him to live only because of the grief it would cause Marianne.