David Sinclair is one of Lord Sinclair's children. When his father was murdered, he was one of the suspects. He used to live in Camelot castle, but when King Arthur and his knights arrived, Lord Sinclair and King Ulthas agreed that the knights had greater need of the castle and so the Sinclair family moved to Sinclair Mansion instead. This greatly angered all of his children. Like his siblings, David has a silver-coated item, in his case a book.
He apparently has little respect for people or any anger management, bullying the mansion staff, being rude to the investigator brought in to solve the murder, and claiming his father deserved to die. However, the staff would not report his behaviour as Lord Sinclair would likely kick David out of the house to protect servants who were there long before he came around. This behaviour caused the family butler Hobbes to consider him the prime suspect. At some point, he bought some poison from the Poison Salesman to clear out a nest of spiders. If there are still spiders around the nest, he is the murderer.
He later conspired with his siblings to get Camelot back from King Arthur and the Knights of the Round Table. They allied themselves with Morgan Le Faye, hired some Kinshra knights from Lord Daquarius, and stormed the castle, taking the knights and the wizard Merlin prisoner. They also threatened the staff, telling them that they would die if they left or spoke to anybody not of the family. They then framed Anna for their father's murder, trusting that the investigator would get her free, and moved out of the mansion and into the castle. After the knights managed to free themselves, Merlin scared them off and threatened to turn them into rabbits if they tried anything again.
^Gossip, "King's Ransom", RuneScape. "The mansion is quite new. Originally, the Sinclair family lived in Camelot, although it went under a different name then. But when King Arthur and his knights appeared, old Lord Sinclair agreed with King Ulthas that the knights had greater need for the castle than the Sinclair family. So, Lord Sinclair had the mansion built, packed his family up and vacated. His children were furious with him for doing it. They felt themselves far better than the knights and felt insulted at having to live in a stuffy mansion."
^Hobbes, "Murder Mystery", RuneScape. "The man is nothing more than a bully. And I happen to know that poor Lord Sinclair and David had a massive argument in the living room about the way he treats the staff, the other day."
^Gossip, "Murder Mystery", RuneScape. "David... oh David... not many people know this, but David really has an anger problem. He's always screaming and shouting at the household servants when he's angry, and they live in a state of fear, always walking on eggshells around him, but none of them have the courage to talk to his father about his behaviour."
^David, "Murder Mystery", RuneScape. "Are we finished now, or are you just going to stand there irritating me with your idiotic questions all day?"
^David, "Murder Mystery", RuneScape. "I don't really know or care. Frankly, the old man deserved to die."
^Gossip, "Murder Mystery", RuneScape. "they live in a state of fear, always walking on eggshells around him, but none of them have the courage to talk to his father about his behaviour. If they did, Lord Sinclair would almost certainly kick him out of the house, as some of the servants have been there longer than he has, and he definitely has no right to treat them like he does."
^Hobbes, "Murder Mystery", RuneScape. "David definitely used the words 'I am going to kill you!' as well. I think he should be the prime suspect. He has a nasty temper that one."
^Merlin, "King's Ransom", RuneScape. "With Morgan cunning and their riches, they make a formidable team. They bought the services of the Black Knights from Lord Daquarius and overran Camelot. We were outnumbered and taken by surprise. The Sinclairs stole our home, but Morgan struck the greater blow by stealing the Holy Grail, Arthur's most prized possession."
^Hobbes, "Murder Mystery", RuneScape. "I'm under strict orders not to speak to anyone not of the Sinclair family. I was told if I left the house I would be killed!"
^King Arthur, "King's Ransom", RuneScape. "I arrived back just as Merlin had kicked out the Sinclair children. He scared them off with a few puffs of smoke and frogs. He told them if any of them left their house again, he'd turn them all into rabbits."