Daniel "Danny" Pennington is the son of April O'Neil's boss Charles Pennington in the first movie. He joined the Foot Clan and was involved in the crime spree. He was seen passing a stolen wallet to a Foot Ninja.
While visiting with his dad in April's apartment, he stole some money from her wallet. Later, he was seen cuffed and taken away by two NYPD officers during April's report. He may have tipped the Foot off that she'll be in the subway. This also gave NYPD Chief Sterns the leverage over Danny's father, Charles, to keep April from butting into his duties under the threat of putting Danny in Juvenile Hall or Riker's Island Penitentiary if April did not leave Sterns alone.
In another visit the next day, he spots Michelangelo hiding under the kitchen table from the mirror. When he turned around he was gone, replaced by a stuffed panda.
On the way to his school, he got into an argument with his dad over him trying to steal a car stereo (which may have been the reason for his arrest earlier). Believing his dad doesn't care about him at all, he ran away, finding refuge at the Foot's hideout at the east warehouse with the other teenagers working for them.
When The Shredder tells the Foot of the Turtles, who must be stopped, Danny loyally tells the Shredder of their whereabouts. He soon regrets it after seeing April's apartment and antique store become engulfed in flames from the Turtles' battle with the Foot as the NYPD and FDNY arrive to battle the blaze and evacuate the area. After seeing Tatsu beat up several clan members for their failure, he begins to question himself about joining the Foot in the first place. He discovers Splinter can talk and the two began speaking to each other.
Casey and the Turtles return to the city sewers, where they find Danny. He's been hiding in the Turtles' lair for days. Danny convinces April not to call his father. While trying to sleep, he has nightmares about the Shredder and Splinter playing tug-of-war with his conscience. He wakes up and heads for the Foot headquarters to look for Splinter.
He finds Splinter and they talk; mainly about Splinter's family, his past and first meeting with Shredder. After Splinter tells him the truth about Shredder, he decides to leave the Foot for good. He and Casey Jones free Splinter, and convince the other teenagers working for Shredder to defect and make amends for their previous actions while serving the Shredder. Danny returns the money he stole from April. However, since she was unaware of the theft to begin with, she is unsure of this gesture, with Danny simply remarking "I owe you". He is reunited with his father, and the two reconcile.