The Doll was a mime that went mad after killing his parents, and sealed his own conscience away out of guilt. Eventually, he came into Marik's mind control, and due to his crushed mind, became very easy for Marik to bend to his will. Marik programmed Doll to kill the Pharaoh at all cost should he win the duel against him.
At first, he appears to be a stationary mime in the park and is a curiosity for many of the people there, but when Marik takes control of him, he goes to confront Yugi and duel him.
Doll has a huge advantage up his sleeve, the Egyptian God Card "Slifer the Sky Dragon". Using a five card combo involving "Slifer", "Revival Jam", "Infinite Cards", "Jam Defender", and "Card of Safe Return", Doll puts the Pharaoh in a tight situation. Every time he attacks, Strings can redirect the attack to "Revival Jam", which regenerates itself after being destroyed and thanks to "Card of Safe Return", he draws three cards from his Deck whenever this happens and because of "Infinite Cards", he can hold as many cards as he wants. With Slifer's attack and defense proportional to the number of cards its controller holds, and with "Revival Jam" able to absorb and reform from any attack Dark Yugi is able to make with the aid of "Jam Defender", Marik boasts the strategy is unbeatable.
Dark Yugi is just about to give up until Kaiba arrives and encourages him to fight on. The two manage to figure out the weakness of this combo and turn it against Marik. Yugi attacks "Revival Jam" with "Buster Blader", and then uses "Brain Control" to take control of "Revival Jam" as it regenerates so it regenerates under his control, forcing "Slifer" to attack it with its second mouth (which is actually "Slifer's" effect of depleting 2000 points from a monster that's been summoned). Then "Revival Jam" is destroyed and regenerates, forcing Doll to draw three more cards due to "Card of Safe Return". Then "Slifer" must attack "Revival Jam" again, since it reformed under the Pharaoh's control, destroying it and allowing it to regenerate again, and forcing Doll to continue drawing due to "Card of Safe Return". This continues in an infinite loop until Strings' Deck is empty, and thus he loses the Duel through a Deck Out. Yugi gets "Slifer the Sky Dragon" for his victory.
Doll himself is entirely devoid of personality, and is merely a shell for Marik's will. This worked to Marik's advantage: since the Egyptian Gods normally do not obey one without a Millennium Item, Doll's empty mind allowed Marik to bypass this restriction: "Slifer" obeyed Marik's will, though Marik didn't have to be there in person. However, Doll's lack of mental awareness also served another end: he had no conscience and could feel no pain, so would hold nothing back. After Strings' loss to Yugi, Marik decided to talk through Strings one last time then released control of him and he collapsed.