It is Cosmo's nickel. Cosmo, has a strong, but strange attachment to this nickel and treats it like a person and likes it for some reason.
Cosmo first found her in the episode Spaced Out when he and Wanda poofed up Mark Chang from Yugopotamia, after Timmy wished for an authentic alien to recreate a scene from Crash Nebula. When the alien's parents found out that their son was transported to earth they were outraged and resolved to destroy earth and rescue their son, not necessarily in that order.
When Cosmo found out that an alien armada was headed to earth to destroy it he called Timmy over to the fish bowl. Wanda told Timmy that they had good news and bad news. When Timmy asked what the good news was after a bit of dialogue, Cosmo replied, "I found a nickel!" despite their situation.
Later in the episode it is revealed what Cosmo named the nickel when Timmy said that he had good news and bad news. Cosmo followed up by saying "I named my nickel Philip!" King Gripullon, Mark Chang's father, asked "What's the bad news?" Cosmo replied "It's a girl Nickel!"
At the end of the episode, Vicky found Philip, which made Cosmo extremely distraught. He was able to reclaim Philip though, because he is shown with her in later episodes.
Philip seems to have a comforting effect on Cosmo. Cosmo sleeps with Philip and sometimes brings her with him when he is afraid, such as in the event of a burglary. He also has a portrait of her in his room right above his bed.
It is possible that Cosmo only likes Philip because of her appearance. He likes nickels because of their luster. And in-fact bet Wanda in a bet with Juandissimo where if Juandissimo lost, he would give Cosmo a nickel. It is also possible that Cosmo likes any other nickel he finds because he only likes nickels because of its shape and other unknown reasons.
Philip is a Nickel
Philip seems to be there for Cosmo, although she was in Vicky's hands for some time. None of the other characters interact with Philip, or see her as anything more than just a nickel. To Cosmo she is special, but the reason for the relationship is murky and no one truly understands it. Wanda usually tolerates or ignores Philip's presence. She seems indifferent to her, also Cosmo's obsession with Philip (or any other nickel) has an effect on his relationship with Wanda.
- Her last appearance was in Hassle in the Castle.
- In Hassle In The Castle, she is shown to have good tracking skills and helps Cosmo and Timmy reach the lobby just before Maryann.
- Coins don't have a gender, but Cosmo just decided to make it a girl nickel anyway.
- The name comes from an episode of Rugrats, Together At Last where Lil, who had lost her bow and was later mistaken for Phil, finds a nickel. Phil the girl nickel is a hidden reference to Rugrats.
- Philip is usually a boy's name, but Cosmo just made it a girl nickel.