At first glance, Caster fits a pretty strong stereotype in Japanese media. Donning fox ears and a fluffy fox tail, it is easy to assume she's a cheerful and adorable character. That's only slightly factual!
Caster tends to be easy going and playful, often breaking the fourth wall by referencing the video game reality and meta information (things the player would only know). It's easy to see that she is aloof and carries the least amount of baggage with her role. This is her first impression to others, controlling the mood with her perky attitude. Her Master will be on the receiving end of confessions of love, flirtations, and teasing - all while they're trying to push forward in a gauntlet-like dive into endless dungeons against other magus. Without any doubt in her confidence, Caster does not fear the impending battle rounds that would delete the user from existence in Moon Cell (the digital, artificial grounds for the Holy Grail War by SE.RA.PH). Bouncy, friendly, and energetic, Caster rarely takes a situation seriously unless those she cares for are in immediate danger. Even then, it takes a lot to shirk her positive aura, focused primarily on fulfilling her role as a Servant and that's to bring her Master a victory.
Her attitude merges with an odd grace. She is aware that there is a time and place for everything - even if she has a habit of showing her ditzy side. As mentioned, she is ferocious in battle but she is also very calculated. When she is actually calm, she has a traditional domestic attitude akin to a Japanese wife in historic Japan. It is important to understand that her true identity is from this era; therefore, this can be considered as a side she shows those she is comfortable with. That reinforces the idea that her "ditzy" persona is merely just a first impression. Her friendliness isn't just locked to her Master, however. She is quick to use her domestic skills to better integrate into human society. Born from divinity, Caster is foreign in her past life - a mortal whom captures the heart of an Emperor and his people. She becomes rather submissive in the sense that she partakes in simple pleasures like serving tea and tending to their needs. There is nothing more exciting for her than appeasing those she's in the company of (as long as they are in friendly standing). To her, humanity's passion and ambition is to be admired and it makes her strive to be polite and act in an acceptable manner to strangers/acquaintances. She wants to integrate into that lifestyle.
However, Caster seemingly takes this less serious than needed, but in reality, she keeps this carefree attitude for the benefit of those around her. Described as being a "perfect wife", Caster strives to please her Master at all opportunities. Defending them in battle, giving kind words and advice, and offering her endless devotion are a few things Caster does on a daily basis during the events of Fate/Extra. Truthfully, she has great concern and care her Master - beyond what a Servant would for their Master.
Like any good wife, Caster cares deeply about those who she is close with. This ensures that she is not only putting others first in her mind, but she is also fiercely loyal. Growing attachment only brings her to be territorial, jealous and defensive of those who are too close to her Master. All of her actions in Fate/Extra are willingly done in the best interest of her Master. Caster attempts to lighten the mood with her bubbly attitude, often flirting with the Master and giving them the option of pursuing a romantic relationship with her (regardless of gender). Truly devoted, she tears down most walls that she keeps up to protect herself, letting those who she values see her as more than just a Servant.
As mentioned, being considered a kitsune, she has some shared mannerisms. Very skillful in a hunt, she is playful and mischievous in her own right. When battling against Dan and his Servant (Robin Hood), she is eager to advance through the dungeon with the desire to hunt him down for poisoning the arena. In addition, she is shown to be quite excitable at the prospect of hunting monsters.
Her role as a fox-hero is actually bittersweet. She does not view herself as heroic or worthy of praise in her past life. It actually is a huge trigger for her. She distances herself from that life and focuses now on making the best for her role as a heroic spirit. For example, it is important to understand she only 'bares her fangs' when intimidated or prompted to. Acting more human than fox, Caster walks a line between both kitsune and typical Servant.
Caster strives her best to... well, be her best. During the events of a previous Holy Grail War, she attempted to make sure her Master won the Grail only to fall short on multiple occasions. After her Master contracts with another Servant, he won and left Caster feeling unworthy of her status and insecure of her skills. Her eager attitude and outgoing will to please her current Master is rooted by her fear of rejection. This also ties in with her past life as Tamamo no Mae (it also suffered rejection by society), which will be referenced below in her back history.
Due to her past, Caster does not like to touch too deeply on her past. In Fate/Extra, information can be a deciding factor of victory and defeat - knowing your enemy more gives you a fighting edge. However, it goes much deeper than that... It's a subject that is very sore for her and weighs heavily on the usually optimistic vixen. As expected by a fun character, she gives vague and nonsensical responds in hopes of rebounding the conversation to something less serious. That's not to say that she will just completely deny requests, but that level of trust is very sacred to her. Once again, this reflects that she inadvertently allows her cute personality wear itself like a mask. One important fact to consider is that despite how horrific the events of her past life is, Caster still doesn't let it pull her down into depression. Those feelings are put at bay by smiling through the explanations and revealing - instead discussing it casually.
Known as a girl Tamamo no Mae (who actually is part of the goddess Amaterasu), she became a lady in waiting to the Emperor. She is not a victim, however. Once she was exposed as a kitsune, it aligned with the sickness of the Emperor. Forcefully exiled and now hunted, she brutally defended herself by slaughtering the first wave of hunters. By the time of her death, she had killed 80,000 soldiers all in defense. That's obviously going to be a triggering topic, right? Her life as Tamamo no Mae is a subject that remains reserved for only the most trusted of souls. It remains one of the huge hurdles someone must overcome to get to know Caster in her entirety - it will also signify that she is worthy of investing her love into this person fully.
Caster is a Servant unlike any other. With a personality that spans from carefree, serious to downright fiery, it's easy to see that there is more about her than a first glance may hint. She is truly more complex than lead to be believed, which is actually quite advantageous to her. After her mortal life ended in tragedy, her role as a Servant is best spent trying to help lift her Master up. She is more than a bubbly, cute girl that offers meta information to the player - she is a knowledgeable and dedicated woman who wishes for nothing more than to please those around her.