Sydney

 is a relic hunter the player will meet when hunting for the Declaration of Independence. If she survives the quest and gets to Rivet City, she will retire from relic hunting and become an ammunition vendor in Underworld. If you get into a fight in the Underworld, Sydney will fight alongside you.

Sydney will sometimes be found dead in Anacostia Metro Station during her post-quest journey from Rivet City to Underworld; apparently killed by a Raider. Though the task is painfully dull, it is advised that you escort her (if you find her alive in Anacostia Metro Station) if you want to have a sure opportunity to buy ammo from her later.

In the Statesman Hotel (Mid Level) (rescuing Reilly's Rangers) you'll find a Holodisk from her Father. Show her this Holodisk and she gives you her Sydney's 10mm "Ultra" SMG and a supply of ammo.

(Note: There is an exploit where the player can repeatedly select the option to give her the holodisk, this is done by following the action above, then reinitiating conversation with her. The player then selects "Since we're done now, why don't you tell me about yourself?" >"Nice gun. I've never seen one like that." This makes the "So, whatever happened to your father?" option open up again, which once selected gives the option to give her the holodisk. This glitch has been experienced in the Xbox 360 and PS3 versions, however it is unknown whether or not it is present on the PC.

Equipments
Sydney's 10mm "Ultra" SMG (unique SMG, 35 DAM initially, 50 DAM at 100% CND) There is also an ammo supply in the Ninth Circle, that belongs to Sydney. It requires her key.

Apperances
only appears in Fallout 3.

Behind the scenes
is based on character Sydney Fox from the TV show Relic hunter.

Glitch / Exploit
she will leave the area and usually is nowhere to be found (not everytime). It is also possible by refusing her proposal to bug the Stealing Independence quest.
 * It is confirmed that If you do not talk to Sydney prior to finding the Declaration of Independence she will disappear from the hall at National Archives. You will still get the mission to fend off mutants if you approach her position (The Super Mutants will spawn.) She won't be at Rivet City / Underworld or somewhere else after that.
 * When speaking to her in the Underworld to get the Sydney's 10mm "Ultra" SMG and ammo, as long as you don't end the conversation, you can ask about her father again, and get the conversation option to tell her about her father again, and get another SMG. Suspect you can continue to do this indefinitely as long as you do not end the conversation.  Note that switching from one instance of Sydney's 10mm "Ultra" SMG to another in your inventory has been noted to cause the game to crash for some people on the Xbox 360 version.  Additionally, exploit may be used even after the conversation is ended, as long as she is still in Rivet City.
 * After dealing with supermutants in National Archives and then refusing her proposal to cooperate

Exploit

 * If you wish to keep her as an extra NPC you can do so easily. She will stay with you as long as you do not complete the Stealing Independence Quest. So do not return to Abraham Washington. A few drawbacks with her as an NPC. She cannot be equipped with any new weapons or armor, she cannot be given any items to hold, she can die permanently, she cannot be told to change tactics or to wait (if you are holding the Declaration of Independence), she has to be manually healed via dialog and she has the health of an average human raider and can be downed quite easily. Overall a decent fighter but more trouble than she's worth.  However, she is handy in overcoming the 1-humanoid NPC companion limit - in one case, a 4-man team was made with the Vault 101 Dweller, Sydney, Dogmeat and Charon.