Who is Luicious Appleby from All About Love and what is their importance?

Luicious Appleby is secretary to Cedric Fortemain. He first appears in the novel asking Horatio's butler, Bristleford about Horatio's death. He is described as having fair good looks, and being popular with the ladies. He leads what appears to be a normal life in Colyton, but none know much about his past.

Luicious was a boy who grew up poor. He went into the infantry and befriended a man who read to him a letter about a valuable painting that his father had. Luicious concocts his plan from there. He murders and steals and does everything in his power to get his hands on the painting, even after it is sold.