I hope that further down the line we can get a Kitsune (not the furry kind) companion. Could be skilled at hiding their ears and tail to hide out among humans and had been intending to contact you anyways (thus available at start).
In my mind it would be a Easy (?) (actually scaling difficulty, gets harder to handle the more tails (power) she gets) difficulty companion.
Potential content would be Corruption, Mind Control, Transformation, Futanari, Gender Bender. Possible classes could include Miko (Shrine priestess with empowered healing magic and anti-demon offensive magic), Gumiho (empowered charm magic (avoid battles, disable enemies) and Energy Drain), Spirit (give up physical body to merge power with PC) and Goddess (requires all 9 tails).
In combat Offensive Stance has her lend her magic to damage the opponent, Defensive Stance has her use her Magic to Heal/Buff, Neutral has her use her magic to try to charm the enemy to cease fighting.
Her goal would be to use the PC to help repopulate her species as they have been getting extensively hunted for their preference to hide in human cities. If PC is male or female and sub the Kitsune plans to turn them into a futa kitsune more concerned about breeding/being bred. If PC is Male and Dom the kitsune plans on becoming their devoted wife. If PC is Female and Dom the kitsune plans to turn them into a futa kitsune and become their wife. Either way she plans to be having lots of kids with the PC.
The Kitsune would offer sex often with the PC, and the PC may find times where they wake up to the Kitsune having their way with them, this allows the Kitsune to build up Sexual Energy which increases their tail count and options (such as charming that group of Guards to ignore you) but if she has too many tails relative to the player level it also makes her harder to handle. In the sense that trying to dom her is less effective, and being sub to her is vastly more effective (totally not a case of her subtly charming you to become subservient to her and feed her more power right?). Accept her advances too much, let her get too strong, and you walk down the path of the Bad (Yet good?) End where you give up your quest and become her breeding machine, and maybe possibly drag down other characters with you.