Charlie was a homeless man who lived in a cardboard box that April gave him, which came with a stove she had purchased. Shadow loved playing in the box and was furious that April had given it away.
Following Shadow's having gone missing, the Turtles decide to investigate Charlie to see if Shadow had gotten away to attempt to play in the box. Upon arriving at the box, they find that Charlie has been murdered by a sai, leading the Turtles to deduce that he had been killed by ninja and that he probably saw something he shouldn't have - which very likely would corroborate with Michelangelo's claim that Shadow was kidnapped. TMNT #7