Astral was found by Fortis in Ultra Space as a Cosmog. With her help they were able to use their abilities to return Fortis to her home universe, and have since accompanied her. They evolved to Cosmoem when protecting Fortis from an attack by a Celesteela, and to Lunala at the Lake of the Moone after being stolen by an alternate timeline version of Pianis and Jenna, who thought that Fortis having access to a legendary's power would only lead to disaster.
Astral is gifted with transcendental power and knowledge, yet often feels lost and confused. Most customs of human and Pokémon society are unknown to them. While they know that they will have to one day leave Fortis behind, they are very anxious about doing so, and would prefer to stay with her as long as possible.
Fortis refuses to use Astral for battle, as she believes that their tremendous power should be used for a noble cause. Astral agrees with this. However, Astral is also highly adept at teleportation, able to open rifts between different universes, navigate the pathways of Ultra Space, and transport people across tremendous distances in the blink of an eye. Fortis occasionally uses them for an easy transport, particularly in emergency circumstances.
Astral thinks of Fortis as their mother. Fortis has happily embraced this role, as she tends to think of her team as her children anyway.