Who is Simon Allcourt-St. James from A Great Deliverance and what is their importance?

Simon Allcourt-St. James is an independent forensics expert who once worked for New Scotland Yard. In spite of having been injured in a car accident in which Lynley was driving drunk, Allcourt-St. James remains one of Lynley's closest friends. The accident left Allcourt-St. James "a cripple," while Lynley walked away without a scratch. Allcourt-St. James is married to Deborah, the daughter of one of his servants. Physically, Simon is extremely thin and not very attractive. He has dark curly hair which is described as "unruly" and he has eyes which are "the satin blue of a highland sky." Allcourt-St. James has hands resembling "the sensitive tools of an artist." He is good-natured and very good at what he does. Allcourt-St. James works with Lady Helen Clyde. It is implied that at one point in time, he and Lady Helen were romantically involved. It is said that Simon Allcourt-St. James is "a man who had known too much pain and sorrow at far too young an age" (pg. 35.)