Jimmy

Jimmy is shown to feel very little guilt for his actions, being more likely to block out things than to feel bad for what he had done. He tends to act bitter, spiteful, passive-aggressive, and in some cases actively manipulative. It had been mentioned that Jimmy experienced difficulty back on Earth; this was most likely the reason why he felt some comfort in his role as co-pilot — he likely felt as if his life was out of his control. Even then, Jimmy still saw himself as "climbing the [corporate] ladder" while imagining Curly as "already [being] at the top". Because of this, Jimmy is envious of Curly's position as captain; he desires the authority that comes with the job, but fails to consider the numerous responsibilities and duties that someone in Curly's position would have. This failure to consider is what overwhelmed Jimmy, and he expresses disappointment with the other crew members — Anya in particular, when she asks for his help with Curly's painkillers. Ultimately, he realizes that he had always been unfit as captain, and that Curly should have retained his position.

Jimmy tends to act impulsively with little thought behind his actions, exemplified by causing the crash of the Tulpar which left Curly grievously injured and fully incapacitated. Prior to the crash, he also immediately begins to panic over learning that Anya had become pregnant after he had sexually assaulted her, running away from the conversation instead of assuming responsibility for what he had done. After the crash, he made an empty promise to take responsibility, yet failed to acknowledge that his actions were what caused the crash. Instead, he accused Curly of causing the crash, and redirecting everyone's misguided hatred onto him. He tends to twist the meaning of other people's words without understanding their meaning, applying them to himself to justify his actions. As a consequence, he becomes an unreliable narrator.

His status as an unreliable narrator is exacerbated by the disdain he feels for those around him, meaning he tends to amplify their negative traits. Jimmy often jumps to conclusions and is accusatory of those around him. He lashed out at Curly at his birthday party after he had told the crew of Pony Express' imminent shut down, stating that he believed Curly wanted to just leave everybody behind. Jimmy often projects his own insecurities and traits onto others, such as when he assumed Swansea wanted to keep the only working cryostasis pod for himself.
(description is from his wiki)

There is so much more about him on his wiki if you need more info for his personality as he is an extremely layered character that we have lots of info about.
For his voice we want him to sound tired and angry like majority of the time. Greasy and Gravely but not to much gravel.