Red XIII, real name Nanaki, is a red lion or wolf-like beast and one of the playable characters in Final Fantasy VII, who also appears in other entries to the Compilation of Final Fantasy VII. Red XIII is the name he received as a specimen designation while held captive and experimented upon by Professor Hojo, and remains the name he is called by the party and in battle in Final Fantasy VII.
Red XIII is a quadrupedal animal with thick dark red and orange fur and a thin brown mane extending partway down his back. His right eye is scarred, and the remaining eye is ochre in color. He is adorned with a variety of beads, earrings and feathers around his head, with golden pastern braces and brands on his legs. One of these brands is the roman numeral "XIII", hence his codename. He also has a flaming tail.
It is never stated exactly what Red XIII is. It is known that his species is very long lived, he had two parents who perished, and he is one of the last of his kind. He appears feline, somewhat similar to a lion, and official sources refer to him as a feline. Reno states in Before Crisis -Final Fantasy VII- that Red XIII "looks like a lion", and in Final Fantasy VII: Advent Children Red XIII can be heard roaring like a lion.
However, his overall movements along with his howling and relation to the moon appear canine, and at another point in Advent Children, he can be heard making a canine-sounding yelp as he hits the ground after Bahamut SIN shakes him off. At one point in the original Final Fantasy VII he is referred to as a dog by a little girl in Costa del Sol the first time the party gets there.
Red XIII is the least featured of the playable cast of Final Fantasy VII in other media, and so at times his character can be difficult to discern. What is certain is that he has a strong sense of honor and courage, especially regarding his father, whom he believed was a coward who abandoned his tribe. He is polite, curious and observant, with something of a distaste for humans which may stem from Shinra's treatment of him. He is highly intelligent, and he knows much about the Planet due to being raised at Cosmo Canyon.
Red XIII can still be feral when called upon. Red XIII's novella in On the Way to a Smile expands on this duality, showing Red XIII torn between living like an animal or a human - his intelligence and conscience makes it difficult for him to live as an animal, while his animal appearance and qualities make him seen as such by humans who disregard his humanizing traits.