The main character in a story must be interesting, intriguing and similar in some way to the audience. Some main characters are at first glance very unusual but the audience relates to them at a deeper level.
A good example of this is the character of Lisbeth Salander in The Girl With The Dragon Tattoo. She is an alternative character, a lesbian goth with a very high IQ, photographic memory and she is probably autistic. However, the audience can relate to her through her value system – a sense of right and wrong – and the unfair way in which she has been treated. She becomes a sort of antihero.
The challenge for the creator of the story is to design a character with enough that is interesting and engaging but without overdoing it so that the individual becomes a parody or pantomime character.
One way to invent a plausible personality is to study the personality tests that are popular in corporate team building. MBTI is interesting because it explains the differences between personality types so an author can scope out a personality without creating the sort of conflicts that make it difficult for the audience to engage with them. Is the character a natural introvert or extravert and how does that make them behave? Are they a thinker or a feeler and what does that mean when it comes to their reaction to events?
Often famous stories have a representative of each personality type in the cast and it is how these interact with each other that gives the story its quality. Joseph Campbell wrote about mythology and personality type and his work had a great influence on some of the most famous stories of modern times, for example the Star Wars series. His work is well worth visiting in to gain a deeper understanding of personality.
There are various psychological tools available that can help a writer to create a consistent and plausible character and understand how personality works will have a significant effect on the quality of the story.