Give them different personalities, different appearances, different ethnicities, different clothing styles, different experiences, different conditions, different morals, different humor, different animations (e.g a shy character will do more shy animations than a confident character would and they might even speak less too).
Personality and traits examples:
- Shy
- Confident
- Bubbly and positive
- Pessimistic
- Loud
- Unintelligent
- Quiet
- Assertive
- Anxious and/or Paranoid
- Dark (as in dark style or dark humor)
- Needy
- Independent
- Intelligent
- Reserved
- Bland
- Hot-headed
- Jealous
- Mean, scheming, cunning
Appearance and styles examples:
- Pretty
- Neat
- Disheveled
- Unattractive
- Attractive
- Hot/Sexy
- Revealing
- Modest
- Plain
- Dark (goth, or dark clothes, or a menacing appearance)
- Liking to a specific color or color scheme, liking to denim etc.
- A lot of jewelry
- No jewelry
- Only important jewelry
- Heels or no heels
Ethnicity examples:
- Ethnic Russian
- Ethnic German
- White European
- Indigenous (this varies so much, Indigenous Australian, Indigenous Alaskan, Indigenous Kiwi, etc)
- African-American
- Thai
- Japanese
- Chinese
- Korean
- Indian
- Mongolian
- Sri Lankan
- Greek
- Latinx
Conditions examples:
- Social anxiety
- Autism
- Aspergers
- Schizophrenia
- ADD
- ADHD
- Schizoid Personality Disorder
- Avoidant Personality Disorder
- Narcissistic Personality Disorder
- Depression
- Allergies
- Anorexia Nervosa
- Asthma
- Bipolar Disorder
Moral examples:
- No smoking
- No drugs
- No drinking
- No teen pregnancies
- No sex until marriage
- No lying
- Smoking is fine, but no drugs
- No smoking, but some drugs are fine
- Smoking is fine and so is all drugs
- No drinking alone, social drinking is fine
- Only soft alcoholic drinks are fine, no hard alcohol
- All alcohol is fine
- Sex whenever is okay
- Only protected sex is okay
- Only abstinence is okay
- (controversial) Abortion is okay/not okay
- White lies are okay, other lies are not
- All lies are not okay/are okay
- Respectable work for money is fine, other work isn’t e.g prostitution, selling drugs, killing etc.
- Any work for money is fine
There’s a lot more examples but I think you get the picture. You can apply so many different morals and views on each character.