Who are the main characters in the twilight book?

There are several main characters in the Twilight series, but it mainly focus' on a love triangle between:

Isabella Marie Swan (Bella)
Edward Anthony Masen Cullen
Jacob Black

Other important characters include:

Dr Carlisle Cullen
Mary Alice Brandon Cullen (also known as Alice)
Esme Anne Platt Evenson Cullen
Emmett McCarty Cullen
Jasper Whitlock Hale
Rosalie Lillian Hale
Charlie Swan
Billy Black

Also, Renesmee Carlie Cullen is introduced in the final novel (Breaking Dawn).

Note that this question did not ask for a list of ALL characters, just the MAIN characters.