Mr. Lindner seems like a nice enough dude at first. He says he represents a kind of "welcoming committee" from Clybourne Park, the predominately white neighborhood where the Youngers are planning to move. Lindner is really polite at first and implies that if people of different races would just sit down and talk to each other a lot of problems could be resolved.
Unfortunately, Lindner's committee doesn't plan to "welcome" the Youngers at all. Mr. Lindner's idea of resolving the "problem" of a black family moving into the neighborhood is to try and bribe the Youngers. He and his fellow homeowners have gotten enough money together to buy the house that Mama bought for more than what she paid for it. Basically, Mr. Lindner and the other white homeowners are trying to do everything they can to keep black families out of their neighborhood.
As the only white character in the play, Mr. Lindner represents the white majority that controlled the country. He also represents the racism of the white majority that segregated America (officially and unofficially) and helped to perpetuate the cycle of poverty which many African-American families had been caught in since the time of slavery.