I think you have to infer a lot of this from what the characters do and say in this novel. For example, with Mr. Harvey, we know he is a serial killer, so his mental characteristics are typical of someone who lures children to their death. Notice when he first runs into Susie before he kills her, how he is very devious and sly - he tries to befriend her, even calls her by name (which she realizes from heaven should have warned her, because he claimed not to know her name at the beginning of their encounter). As the novel progresses, we see how devious he is in pretending to be sorry about Susie's death, eluding the police, and even at the end, where he is stalking another victim. Mostly, his mental characteristics are that he is a pervert.
Susie's mental characteristics evolve over the course of the novel. She is describing the story in retrospect, and she grows mentally. In the beginning, she describes herself as a girl "with spunk" - remember the "Susie peed on Lindsey" story? From heaven, she is upset about missing her family and frustrated that no one can catch Mr. Harvey, but in the end, she comes to grips with her situation and her mental state stabilizes.
If you go through the novel like this, you can infer the other mental characteristics. Think about what the characters do and say in their interactions with each other and how they react to Susie's death. Susie's death, for example, has a different effect on her parents because their mental characteristics are different. Abigail is somewhat unstable and frustrated in her life. She is an educated woman who, with marriage, has had to forsake her career so she becomes disconnected from her family. When Susie is killed, her grief becomes unbearable and not knowing how to cope with it, she has an affair and then leaves her husband. Buckley's mental state becomes one of childhood defensiveness as he attempts to cope with Susie's death, but he is only 4 when it happens. Mostly, his mother's leaving him upsets him mentally and emotionally, and when Abigail returns, Buckley's mental state changes to one of anger and resentment against his mom.
Mentally, Jack feels frustrated and guilty because of what happened to Susie. He feels he has failed to protect her, but his grief spurs him to become actively involved in trying to find Susie's killer. Plus, he has to be mentally strong for the good of his other two children while coping with his own grief. He is strong mentally as is Lindsey, who although she must cope with her grief over Susie (to whom she was close), refuses to live the life of a victim and in the end, she grows into a strong young woman, not an unfulfilled one like her mother.