Original Character Catelyn O'Neill
she is the younger sister of Colonel/ General Jonathan Jackson O'Neill commonly known as Jack. she is often mistaken for being the daughter of Jack due to the 20 year age gap between the siblings.
when she is first introduced to the Stargate program she holds the rank of Captain, and is later promoted to the rank of Major when it is discovered that she contains the ancient gene and has quite powerful control over the ancient systems. most notably of these is the ability to create new ZPMs and restore depleted ones. an ability no other has been shown to share. the process involved in making and restoring zero point modules however is quite dangerous and she can only do so much at one time before she must take a break and restore her health before repeating the process. this however has given the Tau'ri a much needed advantage in being able to explore the universe.
she is in love with Lt Col John Sheppard and he returns those feelings towards her, thus creating a situation similar to that which Jack found himself in with Samantha Carter. a match that Catelyn fully supports.