Experienced in The Sims Bustin' OutEdit
NPC's that move in randomly are characters usually from Continue. This is usually common with The Sims Bustin' Out for console. Sims from Continue are the main character of the game as they are the ones to get promoted and unlock objects for both modes.
These sims can actually move in with sim families in Free Play mode at either Free Street 1, Free Street 2, or Free Street 3. They usually arrive during parties or when friends of Free Street families call them in a specific order (or technically as a random, but unknown cheat).
Sometimes, the character from Continue is the spouse while often times being the main character. This only occurs in the profile chosen to be played in. The main characters cannot be played as, but they treat the lots in Free Play as though they live there. They can bathe, sleep in beds, nap, and cook. They can also make slobs of themselves if they want, without being thrown out by the current residents.
They can even go to work and get fired if they miss three days of days of work. They can also get promoted. However, they will not do any skill building, such as mechanical and cooking. But depending on logic, creativity, body, and charisma, they will not normally skill build unless there's an object there of their tastes.
Main characters may also move in with Free Street families if they are having trouble supporting themselves or the family. They also interact with each other and with the residents as though they've been invited over, too. They cannot get married to Free Street residents if the main character is already is, but will or can have romantic interactions with them if chosen to do so. Usually, the decision is from the NPC.
If the main character is a single man/woman, then marriage maybe applicable. Otherwise, it cannot happen.