As with most novels, characters are extremely important to a mystery book. And some might argue even more so than with other novels. Sure mysteries are generally driven by plot and the "who-done-it" factor not character, but most mysteries tend to be series. And with series, the character is king!
So a quirky character can do a lot for your mystery book. Do they have an interesting talent you could highlight? Something they dislike more than anything?
Things I like to know about my main character before starting:
- Name, age, physical appearance and other general attributes
- His or her interests, status, ambitions, etc.
- Talents and skills
- Tastes and preferences
- Reason for getting mixed up in the mystery
- Background and purpose throughout the story
- Any subplots they might be involved in
- How will the reader be able to relate to them
- Why/how this person became a victim
- How can the reader relate to the victim (the reader needs to want the sleuth to solve this case and care about the victim)
- Suspects name
- Relation to victim and relation to sleuth (if any)
- Apparent motive
- Secrets and lies
- If they did the crime or not
- How you are going to clear them if they didn't commit the crime
- And how you are going to catch them if they did commit the crime
- Relationship to the main character
- Role in the story