Philip the Nickel is a non-living, female nickel that Cosmo found in the episode, Spaced Out. He treats the nickel like an actual living person, which has become a running gag throughout The Fairly OddParents series.
It is Cosmo's nickel. Cosmo has a strong yet strange attachment to this nickel and appears to have somewhat loving feelings for it, despite the fact that it's a non-living inanimate object.
Philip seems to have a comforting effect on Cosmo. Cosmo sleeps with Philip and sometimes brings him/her with him when he is afraid, such as in the event of a burglary. He also has a portrait of him/her in his room right above his bed.
It is possible that Cosmo only likes Philip because of his/her shiny and round appearance since he appears to quickly grow attached to any other nickel he finds. In The Big Bash, Cosmo even unwittingly made a wager with Juandissimo where if Juandissimo lost, he would give Cosmo a nickel, but if Cosmo lost, he would give him Wanda, clearly showing the extremity of his love for nickels.
Philip is a regular non-living inanimate unknown-gendered nickel. Much like any other nickel, she is a shiny, silvery round coin.
Cosmo first found him/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. When Cosmo found out that an alien armada was headed to earth to destroy the planet, 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, and after Timmy refers to Cosmo as "nickel boy," Cosmo protectively hugs the shiny coin and shouts "MY nickel!" before retreating to the fishbowl castle.
Later on in the episode, it is further 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?" Smiling, Cosmo replied, "It's a girl nickel!" However, Phillip is neither a boy nickel nor a girl nickel. He/she is an unknown-gendered nickel.
At the end of the episode, Vicky found Philip, making Cosmo extremely distraught. He was able to reclaim Philip though, seeing that he is shown with him/her in later episodes.
Philip seems to always 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, and it seems that Cosmo's obsession with Philip (or any other nickel) has barely any effect on his relationship with Wanda.