Though her appearances in the novel are brief, Merris Cromwell helps us to see two very important things. First, through her experimentation at The House of Mezentius, she raises the issue of how the possessed (who, in our non-fantasy world could be identified as people who are considered mentally ill or, more specifically, criminally insane) should be cared for.
And second, she helps us to see just how duplicitous Alan can be and how far he's willing to go to help his brother. When Merris learns the truth about Nick and tells Alan to get out of her house, he turns the tables on her, demanding that she find a new place for him and the others to live. Merris replies:
"And why should I do that?"
"Because I still have contacts in the Market," Alan told her. "You may spread your stories about me, and some will believe you, but I'm the sweet, studious boy that everybody likes. You're the mystery. […] And people will believe me. I can make people trust me; you should know that. Even you did." (10.218)
She might not appear on too many pages, but Merris brings some seriously tricky problems with her when she does.