Who is Lady Helen Clyde from A Great Deliverance and what is their importance?

At one time, Lady Helen was in love with Simon Allcourt-St. James. Lady Helen is "Lynley's longest-running mistress" and she currently works as Simon Allcourt-St. James' lab assistant. Lady Helen is stylish and very elegant, as befits a woman of privilege. Not very tall but quite slender, Lady Helen is described as having "a fall of chestnut hair, framing a perfect, oval face" (pg. 27.) Lady Helen is something of a human behavior expert and Lynley calls on her to bring Gillian Teys to Yorkshire in order for Gillian to visit Roberta at Barnstingham Mental Asylum. Lady Helen is perceptive even if a bit on the superficial side. She knows Lynley better than just about anyone. Lady Helen is self-assured and confident.