Sir Leigh Teabing is the main antagonist in The Da Vinci Code, acting behind the scenes as The Teacher, yet helping Langdon and Sophie in their search for the Holy Grail. In the film adaptations, Teabing is portrayed by Sir Ian McKellen.
Background
Teabing was a known historian, whose speciality was in the area of the Holy Grail. It is assumed he was an Oxford student, and may even took part in the Henley sport. Teabing at some point of his life contracted Polio, which caused Teabing to wear metal braces on his legs and use crutches. Although his residence is in France, Teabing often traveles to Great Britian for his medical support, as he does not trust the French health system.
The Teacher
Teabing's first action as the Teacher was contacting Opus Dei's leader, Bishop Aringarosa. Teabing offered Aringarosa a helping hand, as the Vatican informed it will stop supporting Opus Dei. Aringarosa took Teabing's offer to help, who claimed that Opus Dei shall regain its power through obtaining the Holy Grail. Aringarosa commanded Silas, his educated monk, to serve the Teacher, although their communications had to be cut off. The Teacher, without Aringarosa's knowledge, insturcted Silas to kill the four leading members of the Priory of Sion, hoping they will reveal the secret location of the Keystone.
Helping Langdon
After the murder of her grandfather, Sophie Neavue met Langdon, who was recruited by the French police. Through a series of codes and clues left by the late Seaunierre, Landgon and Sophie eventually retrieved an old object, which was rumoured to reveal the location of the Hoky Grail. The couple could not solve the puzzle the cryptex presented, and so the two faced the only source of knowledge - Teabing.
Sophie and Langdon, now convicts after their escape from the Louvre, attracted the police to Teabing's residence.