Mr. Grin is a secondary antagonist in the novel Stormbreaker and the 2006 film of the same name. He is the right-hand man to Herod Sayle (named Darrius Sayle in the film) and also serves as his butler.
Mr. Grin was once a knifeman performing at a circus. His act was to catch knives between his teeth but one night, he was distracted by his mother waving to him in the audience and received a Glasgow smile as a result. He then changed his name to Mr. Grin and became Herod Sayle's henchman and butler.
Mr. Grin aids his boss in his grand scheme to kill every schoolchild in England using a deadly virus installed in his Stormbreaker computers. When Mr. Grin finds Alex Rider sneaking around Sayle's property, he brings Alex to Sayle and threatens him with knives. After being left to die at the hands of Nadia Vole, Alex manages to escape and confronts Mr. Grin on a plane loaded with Stormbreaker computers. Using a stolen gun, Alex forces Mr. Grin to fly him to London so he can stop Sayle. Mr. Grin complies but as Alex parachutes from the plane, Mr. Grin turns the plane around to try and ram him with the helicopter. However, Alex detonates a smoke grenade game cartridge he had left on the plane which causes Mr. Grin to lose control, and the plane crashes into a dock and explodes, killing him.
In the film, Mr. Grin's character is very much the same, although there are a few changes from the book. Alex forces him to fly him to London using a truth serum dart instead of threatening him with a gun, and after Alex parachutes towards the London Science Museum, Mr. Grin does not crash his plane. In fact, his fate in the film is left unknown (although it is possible he was later killed off-screen by Yassen Gregorovich).