Polly was named after Polly Jenkins, the late fiancée of Yanni Yogi, Polly's owner. After Yogi's insanity plea in his trial for the DL-6 Incident, he lost his job and his social standing, and Jenkins eventually committed suicide. The memory of Jenkins was preserved in the name of Yogi's parrot. Yogi would continue to feign insanity and senility in the years to come.
Yogi taught Polly to talk, but she would only respond to being talked to if her name was said first. Yogi taught her to recall the number to his safe. He also taught her to respond, "Don't forget DL-6!" when asked whether the one talking to her had forgotten something. Polly was also able to repeat a previous answer.
Whilst Yogi was living at Gourd Lake, Polly stayed in the shack with him as his only company after his life was ruined due to the DL-6 Incident. She was present, therefore, when Yogi killed his lawyer from the incident. When Miles Edgeworth was set to stand trial for the murder, his lawyer Phoenix Wright and his assistant Maya Fey arrived at the caretaker's shack during their investigation of the murder. Wright and Fey questioned Yogi and were shocked when the parrot mentioned the DL-6 Incident. They also gained the number for Yogi's safe by simply asking his parrot.
Yogi testified in court against Edgeworth for the murder, but when Wright found holes in Yogi's testimony, he ran away, leaving Polly by herself in the shack. Although Maya Fey was eager to adopt the bird, Wright pointed out that the police knew about her and would likely do something about her.
Yogi was eventually captured, but he claimed that he had only gone to get some food for Polly. According to him, she is "a bit of a gourmand" and only eats special high-quality bird pellets from France which can only be bought from a "big pet shop downtown". He testified that he was on his way back to the shack when he was arrested.
It soon became apparent to Wright that he couldn't get any information from Yogi. Prosecutor Manfred von Karma taunted the defense attorney, saying, "Perhaps you'd like to cross-examine the parrot for a little comic relief, hmm?" Wright realized that the parrot was his last hope and desperately called the bird to the stand. However, when asked about the DL-6 incident, Polly didn't respond. It soon became obvious that von Karma had anticipated Wright's move and had Polly re-trained. Luckily, Wright was able to get the bird to identify her owner as Yanni Yogi by saying her name (since she was named after Yogi's dead fiancée) and the number to the safe (which was the date of the DL-6 incident). Yogi soon after admitted his true identity and his crime, having had his cover blown by his erstwhile companion.
"Sayuri" (サユリ) is a common feminine Japanese given name, and was likely chosen for this reason in the original Japanese version of the game.
"Polly" is a stereotypical name for a parrot, and thus it is likely that the names for both the parrot and Yogi's deceased fiancée were chosen for this reason. Using the generic name "Polly" for a parrot may have originated from Ben Jonson's 1606 comedy play Volpone, wherein many of the characters are assigned animal personas that reflect their true nature (i.e., the name of the eponymous main character is Italian for "sly fox"). Sir Politic Would-Be (also known as just "Sir Pol") and his wife are two comic relief characters in the play who try to ingratiate themselves in Venetian society by mimicking the words and behavior of Volpone and his associates, without actually understanding the meaning of what they are doing; Jonson therefore likens them to parrots.
It is unclear what species Polly is supposed to be. Although she bears some resemblance to the scarlet macaw in terms of coloration, she also possesses the large crest common to many species of cockatoo, which the scarlet macaw lacks. It is likely, therefore, that she is not meant to be any species of parrot in particular, although the scarlet macaw coloration is likely due to it being one of the more familiar species to the general public (likely due to it being one of the first parrot species bred in captivity).