Should You Part with Pixie?
The Pixie that joins you at the start of the game will give you a choice to either part with her or not when you reach Yoyogi Park. Below are the effects of letting her go on her own or inviting her to join you.
Parting with Her
Parting with Pixie at Yoyogi Park rewards you with an Ankh magatama. She will then disappear for the entire duration of the game, causing you to miss out on having her in your party and a special event at the late stages of the game.
Note that the Ankh magatama can also be bought at the Ginza Underpass junk shop so parting with her is overall a bad choice.
Asking Her to Join You
Refusing to part with Pixie and asking her to join you allows you to keep her in your party.
Pixie will evolve into Lv. 10 High Pixie at level 6, and into Lv. 56 Queen Mab at Lv. 16 (when your character is at least Lv. 56). She will also learn the useful skill Mediarahan at Lv.57 which fully heals an ally’s HP.
Special Event at Labyrinth of Amala, Fifth Kalpa
You can obtain a Lv.80 Super Pixie by heading to the Fifth Kalpa of the Labyrinth of Amala with Pixie (or any of its descendants that you acquired through fusion). There is a door there that will ask which of your allied demons has been with you the longest. Choosing Pixie allows you to obtain the Super Pixie which has even better stats and abilities.