Alex Walker is the latest main character to join in Series 5.
And was previously homeless and got to know Kazima Tako because she used to give him and the other homeless man he with all the time food and something to drink. Sam also helped him when he was in the streets as revealed in series 6 episode 18 - Me, myself and I.
When we first meet Alex he was thief and none of the kids except for Tyler and Kazima wanted him at Ashdene Ridge but they grew to like him as he got his anger and temper under control.