The blind woman that used to be red johns lover, Harker Rosalind. She has popped up in a few episodes, and has the perfect cover. She's blind, therefore physically implausible to commit these crimes. But what if she wasn't blind? She would be able to commit these murders. She also has red hair. While this doesn't mean that this is the primary cause for her being red john, many red heads have nick names relating to their hair. Harker would be the perfect red john because viewers are shown how incapable she is, requiring meticulous motions and touch just to live out day to day life. Everyone discounts her as a suspect because she can't see. But I think this is a ruse, so that no one suspects her. When Patrick first began making a list of people that red john could be, after the hint from Lorelei, he didn't count women, because an assumption that red john was male was made (this was due to being referred to as 'him', the statistical likelihood of red john being female, and john being a male name). What better way for Harker Rosalind to cover up her being red john?
