Expert Witness
Each side has one expert witness. This witness has specialized knowledge in a certain field (linguistics, forensic science, etc.) that can be used as evidence in trial.
Lay Witness
Each side has lay witnesses, who are people who saw relevant events or know relevant people. Character witnesses often have unique personalities/quirks; the way you act out these roles is up to you.
Defendant
The defendant is the person being charged with the crime, and will either be acquitted or convicted. Just like other witnesses, they add important, often very personal, context to the case. Their alibi/reliability is questioned by the prosecution.