Companion Battie is obtained by reaching a high level of trust with Megaera, and then gifting her Ambrosia.
Using Battie in battle will summon Megaera to aid you. She will appear briefly and deal 2500 damage in a line in front of you. The damage is dealt in small AoE circles, similar to the bombardment attack utilized in Megaera's boss fight.
Battie cannot be used outside of combat encounters, during a Fury boss fight, or while fighting Hades. Hades will eventually grant the authority to use Battie against him.
Like other Companions, Battie can only be used once per combat encounter, and up to 5 times per run (upgradable with Ambrosia) or more if the Night Spindle is bought from the Well of Charon.
'...When Battie first emerged from the darkness, she was overjoyed to learn there was a whole wide world beneath the ground, filled with many wondrous sounds and scents. However, Mother Night instructed her to never fly too far; to stay within the bounds of Tartarus, where it was safe for creatures such as her. Battie heeded the warning, but only for as long as she could stand. One evening, having grown quite restless, Battie flew as far and as wide as she was able, across the meadows of Asphodel, all across the fields of Elysium, such was her excitement to experience it all. By the time Mother Night discovered what she had done and found her, she was thouroughly exhausted. Now, Battie keeps safe whomever she is given to, but can no longer fly as she once did.'
Once Zagreus forges a bond with Megaera, the text becomes "From Megaera, your old flame" or "From Megaera, lover and rival"
It's stated in a conversation with Megaera about Battie that she likes bats, and often looks after their pups.