Son/Brother of Mine was a member of the Family of Blood.
Biography
Having possessed the body of Jeremy Baines from Farringham School for Boys on 10 November 1913, he led his family in a chase to catch the Tenth Doctor to gain his immortality. Otherwise, they would die in three months' time.
After the Family's defeat the next day, the Doctor suspended Son of Mine in time and had him watch over the fields of England as a scarecrow. Son of Mine was the last to be punished, a prisoner of his own conscious mind with a burlap sack draped over his face.
Noticeably, he spoke in a way that made many of his consecutive sentences run together. After possessing Baines, Son of Mine appeared able to keep his eyes opened all the way most of the time. Perhaps as a side-effect from possessing Baines, the right side of Son of Mine's face was nearly always in a half-smile.
He was probably the most intelligent of his Family, which would explain why he took charge of the Family. (TV: Human Nature / The Family of Blood)
Personality
Son of Mine was highly intelligent, as shown by his usage of molecular fringe animation to create an army of scarecrows; He sarcastically explained that he was 'ever so good at science'. He was intensely sadistic, gleefully forcing John Smith to choose either Martha or Joan to die at his hand, taunting Martha as to whether she'd shoot him dead, and at one point calling an opposing army an army of 'little tin soldiers'. He also screamed for the schoolboys to run when they surrendered.
In addition, Son of Mine was very observant, speculating that, even though Rocastle had trained his students to kill and was proud of doing so, the war to come would not have them give him any gratitude. Like the boy he possessed, he was extremely arrogant, seeing no value in humans; when Mother of Mine exclaimed that the Doctor had not only made himself human, but an idiot, Son of Mine muttered 'Same thing, isn't it?'. He was pitiless and sly, as he was unafraid to kill anybody if it meant that he would achieve his goals, almost considering the act of killing to be sport if his victims ran for their lives - he didn't even appear to aim for them while hunting, taking pleasure in their terror.
He was cruel, dry and heartless, eagerly sending his scarecrows into war against Farringham School. In spite of this, he was capable of a cold, childish sense of humour, asking Rocastle if he would like to cane him and if he would like it. Nevertheless, he did seem to have a soft spot when it came to his family, specifically his sister, as shown when he expressed the hope that the Doctor would one day forgive her and free her from the mirror in which she was imprisoned; he did not express this sentiment to either of his parents, but perhaps because it was more conceivable to him that she could be freed, whereas his parents were in circumstances where that was impossible. He also, upon realising how the Doctor had outplayed them, screamed for his family to get out of their ship moments before it exploded. Later, Son of Mine expressed his terror at the Doctor's fury, and the fact that the Time Lord never raised his voice. He voiced the irony that he and his family had sought out immortality, and the Doctor had given them exactly that. (TV: Human Nature / The Family of Blood)
Behind the scenes
Son of Mine is one of the enemies in Doctor Who Legacy mobile game.