Gai is a Konoha Village jounin and the sensei of the team whose members are Rock Lee, Tenten, and Hyuuga Neji. Like Lee, he is a specialist in hand-to-hand combat or taijutsu, but unlike Lee, he can use ninjutsu or genjutsu.
He and Kakashi are known as eternal rivals, gai is in the lead with 50 wins to 49 losses. Later it becomes known that this is just the score of a rock-paper-scissors game they played when they were kids. He is also a fantastic teacher since he really cares about his students and their feelings, especially Lee. Although he really cares about his students, he is very strict about his rules and those who break them will receive a punishment.
Gai and Lee are very close. Lee is Gai's favorite student because of his nindo, (way of the ninja), and because Lee looks like him in his younger days. Gai is also proud that Lee has mastered the renge, secondary renge, and the ura renge at such a young age. Gai especially appreciates Lee because he has the same eyebrows and the same nindo. He sees his childhood self in Lee.
Kakashi: "Did you say something"
Gai Sensei: "Oh my god!!! That was pretty good rival Kakashi. That reaction is somewhat 'modern' and it pisses me off."