Neco-Arc (ネコアルク, Neko-Aruku?) is a Neco Spirit appearing in the Tsukihime series. She is a pint-size cat girl parody of Arcueid Brunestud, acting as the mascot character of Type-Moon at times. In Fate/Grand Carnival, she appears as Mysterious Neco X (謎のネコX, Nazo no Neko X?), parodying Mysterious Heroine X/Artoria Pendragon.

Neco-Arc is a member of the Neco Spirit race, originating from the Great Cat's Village.[2] She became Ms. Ciel's student, constantly acting up in retaliation for having such little screen time and wishing people to think she is smart.[3] She is referred to as the "work of Kischur Zelretch Schweinorg" according to Aoko Aozaki, suggesting she was brought from another world, but it's also said that there is a "certain urban legend" telling of six Neco-Arcs and one that exceeds them that could be a materialization of TATARI.[4]

Other Neco-Arcs include Neco-Arc Bubbles, Neco-Arc Chaos, Neco-Arc Destiny, Neco-Arc Evolution, and Mysterious Neco Z.

Neco-Arc is known as the biggest small-fry in the Cat Kingdom.[3] Most characters are unnerved by her appearance, and she especially torments White Len, calling her a "tsundere", and Satsuki Yumizuka, who Neko-Arc constantly reminds that she doesn't have a major role in Tsukihime. She recognizes Miyako Arima as a much-awaited future rival.[5]

Neco-Arc (Mysterious Neco X in Grand Carnival) and her fellow Neco Spirits run Ahnenerbe, the strange coffee shop that once every ten years allows worlds to converge. Each episode begins with a short ♥♥♥♥ of them at Ahnenerbe, which ends with the Neco Spirits turning to the camera and declaring "It's starting!" before the opening credits play.

Neco-Arc is the second strongest character in the Nasuverse after Magical Amber.[6] Taiga Fujimura would obviously lose to Neco-Arc.[7] She is twice as strong as Arcueid, but she is weaker than Berserker because cats only have nine lives compared to his twelve.[8][9]

All of her techniques are extremely powerful like "Dead Neco-ning" and many more, able to destroy everything in her path.[3] Her eye beams are called the "True Ancestor Beam", a "vaguely familiar blast" that cannot be fully observed with the naked eye.[10] She has a warp skill called "Opuba!". Her uppercut, "Cat Out of Hell", sends the opponent flying into the heavens.[3] Her killer move is I want to go to the Crocodile Garden (ワニ園に行きたいわたし, Wani-en ni ikitai watashi?), where she attacks horizontally by turning into a rocket. Like the rest, its fearsome effectiveness is seriously troublesome, and it may have some connection to the Farewell, Ciel-sensei! Our friendship ended in the Crocodile Garden! (さらば知得留！　ワニ園に散った友情?) technique of her third character popularity poll command list.[10] The Elite Neco Corps (エリートネコ部隊, Erītoneko butai?) summons the GCV's only military unit, but while they boast in power, it is unrelated to its power as a technique. The high degree of randomness in that she cannot control them makes it only useful as a provocation.[11]