[spoilers] Lucas. Just Lucas.

[Belle]

on the Lucas/Sarah relationship: I disliked her from the moment I saw her, but I think it was a way to show to us how 'messed up' Lucas was in terms of being able to have and hold a human relationship (weither it was romantic or friendship), which I think, he craved for immensly! Remember the constant craving for approval and recognition he wanted from Harry, not only to be trusted again, but to feel, or fool himself into believing, he was still able to have 'normal' human relations.

I do believe that it is possible for someone who has been trough such an ordeal (imprisonment, torture), who then came home to find his heart crushed by the woman he truly loved and who was not being trusted by the same people he almost gave his life for, to behave in the way we saw Lucas behaving.
He was so deprived of human feelings towards him, the moment he saw a sparkle of that (comming from Sarah) he threw himself completely at her mercy, not willing to see her (obvious) faults because that might mean he would loose the humanity he thought he was undergoing.
I think he sort of lost the ability to tell friend from foe if it didn't was directly related to his work,
it was only when Sarah lay in that hospitalbed and stated why she agreed with Nightingale's MO and thus joined them, he saw clearly what she stood for and realised it was exactly what he was fighting against.
I believe he didn't grieve for her as a person when she was murdered but for the knowledgement he lost the only thing that gave him the feeling he was still a human.
In S9, the one with the American hacker, he tells her he doesn't listen to music anymore and, as someone stated (forgot who ), his living spaces are decreasingly personal, so, I think, that all leads me to conclude Lucas was struggeling deeply to feel human again after Rusia, and grabbed every straw he could get his hands on; in this case Sarah.
These are merely my thoughts and I appologise if they come across as incoherent!

I do agree with the statement being made that it was all a bit of a pitty that the psychology of this relationship and the one he had with Elizavyeta was sacrificed to provide us with the eyecandy! They indeed should have made more episodes so we could have had both .

I refuse to think any deeper on the relationship Lucas had with Maya because
a) Maya's character was as hollow as a windpipe
and
b) I still can't, and never will be able, to get my head around the Lucas is actually John arch!