Claire was a former Magical Girl, a member of Marianne's team until her demise at the hands of a powerful magico.
Backstory
Claire was an Italian American, born somewhere on the East Coast of the United States. Whether it be by birth or infection is unknown, but she was also a werewolf, something that notably affected her behaviour. Little is known of her personal life, but Claire at some point achieved a PhD in astronomy, and was well regarded in the field.
As part of Marianne's Magical Girl team it can be assumed that Claire would have been both skilled and powerful. Her ability as a tracker was quite noted, seemingly related to her status as a werewolf. It is said that she never forgot a scent, that once she had a trail she would be able to follow through to the target even across planets or even dimensions.
It was during the 1990s that Marianne would form her magical team consisting of Claire, Olivia, and Evelynne, though it is unknown if she knew Claire for a longer time prior to the formation of the team.As part of the team, Claire went on all sorts of both grand and terrible adventures with the team. It is known that during their time on Nera, Claire gained the traits of a bunny. She would eventually die in battle against a powerful magico the team sorely underestimated, with Evelynne being magically crippled in the same battle which would leave her stuck in the form of a man.
As the purported heart of the team, Claire's death was sorely felt by her friends. Olivia seemingly blames herself for her death, and has purportedly never been quite the same afterwards. Even the callous and jaded Marianne of the present day will admit to missing her. The loss of Claire, and being replaced by Phyre, certainly could not have positively impacted the overall mood of the team.
Personality
Marianne describes Claire as having been a foolish idealist, a klutz, and a ditz. As far as being a ditz goes she was reportedly even worse than Natalie, but as previously mentioned she was still intelligent enough to earn a PhD and be acknowledged by her peers in the field. Her intelligence really came into question sometimes though, such as her strange belief that the mafia is associated with werewolves and being put out when people associate her with the mafia thinking it was due to her werewolf nature rather than because of her Italian heritage. Claire's intelligence or lack thereof aside though, she was also in many ways the heart of their team, with her constant smiles and dumb optimism that helped them through the darkest of days.
Beyond her general personality though, Claire developed numerous quirks owing from her status as a Werewolf. She had a tendency to eat food raw, something that led to her having the worst cooking ability on the team, and especially loved to eat rabbits. This habit was particularly bad during her time on Nera where she oddly took on rabbit form instead of wolf form. The Neran natives took her for some sort of cannibal barbarian, though the rest of her team thankfully kept Claire from eating anything sapient. She even had canine habits such as marking territory, which while not too bad in wolf form, she has also done in her human form (which once got Evelynne in trouble when she took Claire home and she tried to mark a fire hydrant).
This duality between woman and wolf worked in reverse as well. Claire's normal behaviour also did not stop when she was in wolf form. Large smiles are much more disturbing when accompanied by a mouth full of sharp teeth, and while headpats from a cute girl might be nice, they are much more frightening coming from a nine foot wolf monster.