Don Logan is a mentally disturbed gangster from London. Don's function in the narrative is an important one. He destroys the normality of Gal's paradise home in Spain, by pressuring him into doing a job he doesn't want to do. Without Don, the film would simply be about Gal's life on the Costa Del Sol.
One scene in which Don is most important is when he returns to the villa after being thrown off the plane. Everything in the scene is focused on his presence. The camera is constantly on him, even when he's not directly in the shot, you can see him in a reflection. The music changes from happy pop when he wasn't there, to a dark silence when he comes back to the villa. There is not much editing in this scene - there are only a few cuts in the action, which increases the tension that Don is causing. Finally, there is a low lighting element to the Mise en Scene that creates a feeling of tension.
This character is rather typical in the British Crime Film genre. He is typical in the sense of his authority, which is shown in his costume and his dialogue towards the beginning of the film. However, it is not often that you see a character with such mental issues as his. Although this is rare, when Harold in The Long Good Friday loses his temper and kills Jeff, the two characters are very similar.