Red XIII is a mysterious character in the game. He has a bit of a stronger importance than some other characters, but he's not too important. He's from Cosmo Canyon, and wants to follow in the steps of his cowardous father who took a brave turn in life; Sato, his father, was turned to stone by poisoned arrows shot by Gi, who was threatening his family and town. Red XIII, also known as Nanaki by his grandfather, is also one of Hojo's experiments (number 13/XIII). He was rescued by Cloud and company and decided it'd be best to join his team. Red XIII wants to save the planet to become strong and make his fellow villagers proud.
Red XIII is very weak in HP. I don't use him often, but he might seem useful to others. I think it's hard to find a weapon for him stronger than comparing a weapon for Tifa or Barret, but maybe that's just me. Also, his MP doesn't seem all that great, like Cid's or maybe even Barret's, and I don't think his strength is very good to begin with. But I just don't play with him often.
Red XIII's Limit Breaks
Red XIII's Weapons