The Lab Rat

Any character who performs scientific analysis. Forensics people, usually. If on a show about those people, such as CSI and its spin offs, he will be the member of the team who deals with the most technical and specialized aspects.

They will often be fairly geeky, and/or members of other subcultures. Not looking out of place on the catwalk is optional. Sometimes they will leave the lab and have adventures, or even become a more major character. Often the Plucky Comic Relief if they get any character development.

Anime and Manga

 * Any character in the Mazinger series (Mazinger Z, Great Mazinger and UFO Robo Grendizer) filled The Professor role fit in this trope: Dr. Kabuto, Prof. Yumi, Prof. Kenzo Kabuto, Prof. Umon... All of them performed scientific analysis and research, either to design the Humongous Mecha or to draw the information they needed from the data the Bridge Bunnies collected.

Literature

 * The Dresden Files has Waldo Butters, whose adventures outside the lab result in him very firmly returning to it, and only leaving while wearing a Bulletproof Vest.
 * In the Palace of the Prophets, Wizard Warren becomes known as The Mole for stopping leaving the underground libraries. It takes Richard to draw him out again. Warren turns out to be both brilliant and a power combat-wizard, so it was mainly shyness.

Fanfic

 * Dr. Otto Octavius refers to himself as exactly this in The Life After Death Trilogy.

Live Action TV

 * Nigel Townsend from Crossing Jordan. He was as much a main character as anyone who wasn't Jordan could be throughout the show's duration, but his overenthusiasm at tackling any overly technical problem, quirky sense of humor, ambiguous personal life, and arguable ability to fall into the goth subculture place him squarely in this trope.
 * Greg Sanders from CSI, whose nickname at Television Without Pity inspired this entry's title. He has now become a CSI in his own right.
 * His role has been taken over by multiple people- David Hodges (Trace Analysis, endearingly annoying), Archie Johnson (Audiovisual) Mandy Webster (Fingerprints), Henry Andrews (Toxicology) and Wendy Simms (DNA, lower rung love interest). They got a Lower Deck Episode with the hilarious "You Kill Me".
 * Also, the aptly named "Lab Rats" episode, and the(also aptly named) "Field Mice".
 * CSI: Miami had Natalia Boa Vista, who has upgraded to full CSI.
 * Maxine Valera also does this job.
 * CSI: NY has Adam Ross.
 * Marshall Flinkmann, the Mission Control from Alias.
 * Broots from The Pretender, whose job description rarely entails leaving his electronics lab at Villain HQ.
 * Abby Sciuto, the Perky Goth lab tech from NCIS. A major case of Bunny Ears Lawyer.
 * In Bones, either Zack, Hodgins or Brennan (in that order or probability) depending on the episode and the context.
 * Called "squints" in-universe by Booth. Because they sit in the lab and squint at things.
 * Also the rotating team of 'Squinterns', Dr Brennan's interns.
 * While "serial killer" is a better description, blood-spatter analyst Dexter Morgan is considered The Lab Rat in his public life.
 * If anything, Dexter is a subversion. At his job, he's seen as one of the lab rats, but the audience knows better. His co-worker Masuka, on the other hand, is a dead-on example.
 * Chloe O'Brian from 24.
 * Samantha Carter from Stargate Verse.
 * Fred (and later Knox) in Angel.
 * Early seasons of Buffy the Vampire Slayer have Willow doing this, before it becomes less "chemical analysis" and more "potion-brewing".
 * In Dollhouse while Topher has pretensions of being a Mad Scientist he's really just one of these, his assistant Ivy even more so.
 * Agent Pendrell in The X-Files.
 * Lester in Beakmans World is a literal lab rat. Well, lab guy-in-a-rat-suit, but you get the picture.
 * Eve Lockhart in Waking the Dead also qualifies.

Video Games

 * Ema Skye in the first Phoenix Wright Ace Attorney and Apollo Justice Ace Attorney.
 * Technically, a detective who really likes science, but the spirit is still there.

Web Comics

 * Gunnerkrigg Court has Kat, even when dealing with second shadows and ghosts.

Western Animation

 * The Lab Rat from Grossology actually goes by this name.
 * A sort of literal version of this occurs in Creature Comforts, where a couple of lab rats in a maze being voiced by two real-life scientists talk about their day jobs.