Celest is the daughter of Ace and will appear in Akintale at a later date though she will be seen in a photograph kept in Ace's wallet sooner. Her physical form is a humanoid artic fox, causing her fur to dramatically change between the thick white fur of the winter months and thin brown fur of the summer. Due to her base animal being very small, she is slightly shorter than Sans. She tends to wear a pin striped suit. Her eyes glow yellow, similar to Ace's cyan but are significantly less sensitive to light. She is also codeless.
She is usually outside of the Undertale multiverse so attempting to contact her is often futile.
Celest has no direct combat ability herself and cannot summon any kind of weaponary. She was taught to use a bow and knives by her family but it quickly became obvious that this was not where her strength lay.
She is, however, a powerful psychic that can withdraw any information from the minds of anyone that hasn't specifically learned to defend against her mental attacks. This is because her method of doing so is unique and creating a block would require many decades of trial and error. She actually doesn't have any choice in accessing the minds of people around her and does so is automatically. Because of this, ambushing her is impossible.
If threatened, she will use the information she gained to find the weak point in her aggressor (such as a fear or regret) and induce sensory hallucinations that takes advantage of those issues. For non-sentient beings, she simply uses a base fear such as fire or lightning. These hallucinations are visual, auditory and tactile so seem completely real in the mind of the person the hallucination is induced in. For example, if her attacker had lost a friend or family member and considered their death to be their fault, she will create a perfect image of them exactly as her attacker remembers them and cause the vision to say and do whatever is needed to emotionally cripple her opponent. She has caused multiple beings to commit suicide using this method, not that she cares.