Ghost is Heidi's Shiny Unown. It's an "!" Unown that she received from Izanami a few years ago. Ghost has been her companion and her assistant ever since and has been instrumental to her learning and understanding creation power ever since.
Usually, the Unown share sort of a hivemind together where they communicate with each other and observe the world passively; however, Ghost was detached from this hivemind so it can more efficiently assist Heidi in her training and progression. They consider each other as close friends and they're working together to push boundaries of her abilities both as a Latias but also as an Arceus.
Ghost also possesses Hedi's Rotom Phone. Actually, Heidi made the phone chassis herself and modified it for Ghost to use, but she used the Rotom Phone as a base. With that chassis, Ghost is able to act as Heidi's communication device, but also as a simple way for Heidi to interface with her Arceus abilities and duties. They make for a good team.