Rin is a hot-headed boy.
He's peevish and straightforward, and he always says to the others what he thinks, without using turn of phrases. This is the reason why sometimes people consider him aggressive and scary, but truly his heart is pure and all his words are sincere.
Rin promised himself to become stronger in order to protect Yukio and his friends, and obviously, because he doesn't want to see another person dying in front of his eyes.
He's Satan's son, but since he's born from a human woman and him, he grow up in Assiah with his twin brother and the step-father, Father Fujimoto, a priest. He found the truth only after his 15 years, because before this time, Fujimoto and Yukio treated him as a normal human, knowing that his real powers will be revealed only when Satana would turn in Assiah to bring his proper son.
Father Fujimoto dies in order to save Rin's life and when he realizes that his father was an exorcist, he decides to follow his steps and to join the Order of the Exorcists.
It's unusual that Satan's son decides to choose a difficult way to fight him, but Rin's desire is to defeat that monster. In a first moment, to get reveange, but, gradually, to change all the bad things he did and to protect the ones he loves.
Since Rin doesn't like to study, he decided previously to find a job, but in order to become an exorcist, he was costricted to join the Academy and to apply yourself in all the subjects, even if he finds a lot of difficults following lessons and schoolworks.
Rin's power is the same of Satan, and it consists in an infinite and negative energy released with blue flames. He has not the control of his body and his power, and he can't stop himself to fight when emotions and intense feelings sweep his mind.
In order to help him to control the flame, Father Fujimoto gave him Kurikara, the demon slayer sword ables to contain his power.
Ris has teeth, ears and tail of a demon, and he always try to hide them leaving Kurikara in his sheath. When he draws one's sword, he assumes the demon form, and obviously, the power is released.
But Rin's power is a weapon that can be used against Satan, and Mephistopheles accepter him in the Order of the Exorcists in order to learn to him the way to control his power, making him the perfect, ultimate weapon to use against Satan.
Rin understands only the minor part of Mephistopheles' plans, and he knows to be used, but he accepts this condition in order to keep his promise.
In fact, is thanks to the Academy and the training with Shura if Rin becomes able to learn to control his flames peace by peace.
His pride and his desire to don't be useless, together with the goal to surpass Yukio, are the motivations that let him goes on, and looking at them, he finds the strength to be meek and to follow the orders of expert exorcists like Shura, even if he feels umiliated showing his weakness.
Rin's desire is reaching and becoming a Paladin, the special rank of the Order of Exorcists. He works very hard in order to don't delude the trust of who believes in him.
But that's not all, Rin isn't just a boy that thinks to surpass his brother and to kill Satan, he has feelings like all the other guys of the same age and he has a crush on Shiemi, the first, real person that he thought to be similar to him.
Her opinion and her smile are things that Rin needs to have in order to be happy.
His hobby is cooking and he's pretty good to cook foods. Funniest moment for him is eating it with his friends, especially with her.
As I previously said, Rin is a person rich of feelings. He loves animales and a lot of kind of creature, and he's always ready to fight against people that try to hurt them.
After the meeting with Kuro, a "special" demon cat that Fujimoto considered his pet, Rin decides to take care of it and they become friends. Kuro can feel Rin's golden heart and this is the reason why it adores Rin, spending the major part of the time upon his head or one of his shoulder.
It's written by me, so my apologies for my bad English...