And you know what? This association of multiple personalities is totally fitting. Because The Westing Game's Sandy is not what he seems.
The Sandy we first meet is the poorly-educated doorman at Sunset Towers. Here are the things we know about him: he tells us he has a wife and children (whom we never meet) and he's pretty open about his poverty/social situation. After working at Westing Paper Products for twenty years, he was fired, which is how he wound up as doorman.
As the Judge's partner in the game, he's the only other player she doesn't investigate; he's also almost pathetically grateful to her when she gives him her entire share of the $10,000 the teams get as playing money. He helps her keep track of the research she's doing on the other players, and suggests that Otis Amber may be the real Sam Westing. When he suddenly dies at the Westing house during the second will reading, everyone's really sorry, especially Turtle—she says they were friends.
Sandy makes a small interruption during the first reading of the will, and nobody laughs at his joke. But they're really not laughing later, when they realize that his joke means they're missing a key word from the will. He actually influences a lot of the action in the book: he pushes Otis to tell the ghost story about the Westing house, he encourages Mr. Hoo to invent paper insoles, he directs the other players to put all their clues together... and he foils Judge Ford:
She was paired with the one person who could confound her plans, manipulate her moves, keep her from the truth. Her partner, Sandy McSouthers, was the only heir she had not investigated. Her partner, Sandy McSouthers, was Sam Westing. (24.32)
And his choice of cover is perfect—as a doorman he sees all the comings and goings at Sunset Towers, but perhaps because he's the doorman we never follow him back to his own house.
But the most defining aspect of Sandy's personality is this: he's a good liar. He doesn't really die in the Westing house, and his name isn't really Sandy McSouthers. He's Sam Westing… in disguise.