Lyn

 is Claire Willams's adopted sister. (Note: If you do not know much about Invader Zim, the following will not make much sense to you. Read at your own discretion.)

Past life
Lyn is not human, she is Irken. She was traveling through the galaxies, where she was forced to go to Hobo 13 by the Tallest, when her ship abruptly ran out of fuel. (She learned later that RiRi, her SIR, emptied it out to make room for her collection of smashed guitars.) Lyn crash-landed in Danville and freaked out, because she hadn't thought out a disguise. After a day of observing normal human behavior, she created her disguise but couldn't set up base anywhere. Mrs. Williams, her current foster mother, found her wandering the streets and invited her into her home.

Personality
Lyn is power-hungry and likes has to be the leader. She has heard of Operation Impending Doom 2 and is unofficially part of it, rather like Zim, who is not really an invader.

Incognito
Lyn's disguise looks vaguely similar to Tak's in that she has mastered the art of disguising herself well, not like Zim, who simply wears contacts and a wig. She has bobbed purply-black hair and steel-blue eyes, and wears a dark purple shirt and black pants, disguising her PAK as a backpack.

Alien
Although Lyn does not normally take off her disguise because her foster family does not know that she is an alien, without her disguise she has bright blue 'Irken' eyes so to speak (like Zim's without contacts) and the typical green skin.

Phineas Flynn
She is somewhat friends with Phineas, but when he fails to answer her questions about obscure planets, she likes him a bit less.

Ferb Fletcher
She is pretty good friends with Ferb, since Ferb is Mr. Smart and knows all about planets, including Conventia and Foodcourtia, both of which are Irken planets.

Isabella Garcia-Shapiro
Isabella has caught on that Lyn is not human, but doesn't say anything for fear of looking insane in front of Phineas. She is a bit skeptical of Lyn and doesn't exactly like how eccentric (to Isabella) she is.

Zim
She and Zim are good friends, and Zim is the only one (besides Dib, who can smell an alien from a mile away) who knows Lyn's secret identity.

Dib
Lyn is as close as you can get to best friends with Dib. She has not outright told Dib that she is an alien, but Dib has put two and two together and guesses that she is Irken. Lyn also has a crush on Dib, but it is not known if he returns her feelings. However, he does show a bit of emotion when he doesn't try to expose her like he does with Zim.

RiRi
RiRi is Lyn's custom-made SIR unit (remember, she isn't officially part of Operation Impending Doom 2). Lyn, however, is horrible at building robots, and so RiRi has a personality rather like GIR's, though Lyn can sometimes (keyword is "sometimes") get RiRi into duty mode, even if it's only for a few minutes. RiRi disguises herself as a bunny and talks to GIR a lot.

Background Information

 * Lyn is voiced by Alyson Young.
 * Lyn's "human" name is Lyn Williams, because that's her foster family's name, but she definitely doesn't prefer being called Lyn Williams, ever.