As Jason was killed prior to the start of the series, not much information about his personality is known. According to Brett, he was the best brother anyone could've asked for, and was a genuinely nice person who didn't deserve to die. Jason was very protective of his loved-ones, especially Brett.
Despite his death, Jason's actions still has a great effect on his younger brother, as his love and affection on him made it possible for Brett to have peace with someone. Brett is also haunted by Jason psychologically as well, consistently having dreams of Jason’s corpse attacking him, almost killing him. Brett himself would go on to feel he was gradually becoming like someone else as the series progresses.