This can be one of the hardest parts. Because in writings written as the product of a single mind, the characters tend to resemble each other, and this is quite normal.
There is an easy way to get around this problem in the first place. For example, think of your main character as yourself and give your own characteristics. Then consider some traits for the best friend character. Have features such as animal lover, who likes to go on a picnic. Then try to find these traits in the people around you. You can identify your friend or acquaintance who reflects these characteristics the most with this character and continue to write on it. Or the selection process could be reversed. You can choose a relative of yours and determine the characteristics of the character through their characteristics.