Matthew Blanchard is the brother of murder victim Katy Blanchard. He was involved in Katy’s fatal overdose, which she was left to die from by the Blanchard family’s financier, Jerome Kamen, in the hopes of draining Matthew.
When Karen was tried, Matthew was blamed for killing Katy due to wanting to bury his own drug addiction, but Kamen took a plea deal anyway. (L&O: "Guardian")