"Are you kidding me? You pass up a cake for two days? What's the deal? What've you done with the real Lou Welch? Security! Security, over here! We got another guy with a changeling net!"
- – Garibaldi to Lou, 2259.
On Babylon 5
Welch came aboard Babylon 5 to serve as a security officer when the station came online in March 2256. In 2258, during an investigation into a possible sabotage of the station's cobra bays prior to a state visit by Earth Alliance President Luis Santiago, Welch was ordered by Major Lianna Kemmer to guard Garibaldi's quarters when suspicion fell on the chief, prompting a station-wide fugitive alert (Survivors).
Later that year, Lou found himself being repeatedly questioned while on his lunch break about Commander Sinclair by a man calling himself Aaron Franks and claiming to be a rep for Quartermaster Corp. Franks was really Colonel Ari Ben Zayn of Earthforce Internal Affairs Division, and his pointed questions aroused Lou's suspicions enough that he reported the incident to Garibaldi. 
In January 2259, an enraged Lou led the arrest team that apprehended Jack after he was revealed to have shot Garibaldi in the back on New Years. Lou suggested that Captain Sheridan take a walk around the block a few times and leave him to them. Though Sheridan appreciated Lou's "enthusiasm", he ordered the prisoner to be put in lock-up under maximum security with checks every quarter-hour. Shortly after Jack's apprehension and Garibaldi's recovery from being shot, Welch and many of the rest of the security officers missed Garibaldi and were quite relieved when he returned to work.
Later that year, Lou was promoted to serve on the personal guard of EA President Clark but later resigned from Earthforce during the Earth Alliance Civil War. Drawing on his experience, Welch found work as a private security consultant for a number of small firms and also gained quite the reputation as a tracker, earning himself the nickname 'The Ghost.'
The real secret behind Lou's success as a tracker was his possession of a piece of technology he acquired on Cygnus IV. At the time, he had been working security at a power plant for an eccentric owner who believed Martians—little antennaed green ones, not the real thing—were trying to take over his factory. While he was there, the planetary sensors detected a ship entering the atmosphere, spiralling down before disappearing off the sensors. Lou led a team to investigate and found a crashed ship unlike any he'd seen before, though it reminded him of one he'd seen several years prior on ISN. Entering the ship, they discovered the body of a grey skinned alien with a terrifying aspect. Telling his men to stay back in case of danger, Lou made a closer inspection of the interior, during which he found what he called the "cloak". A delicately woven, web-like piece of fabric that, while beyond any current tech Lou was aware of, operated similarly to a changeling net. Though instead of overlaying the wearer with another's appearance, it effectively blended them in with the background. Through later experimentation Lou discovered its limitations, including that it worked somewhat less effectively in broad daylight and that it remained deactivated as long as it was folded in upon itself. No one else on the search team saw the cloak and Lou decided not to volunteer anything, keeping the device for himself.
In 2269, Welch was hired on behalf of the Interstellar Alliance by his old boss Michael Garibaldi-now CEO of Edgars/Garibaldi Enterprises-to accompany himself and Citizen G'Kar to Centauri Prime and investigate suspicions that the government might be preparing for another war with the Alliance. During the course of the investigation, Lou was killed by Throk Milifa after Throk discovered Welch in the act of downloading data that proved the Prime Candidates were behind a secret Centauri military build-up. Unbeknownst to Lou, his use of the web had attracted the attention of Shiv'kala, who followed Welch and remotely disabled it so that Throk would discover him. Lou's body was taken out and quietly dumped in alley where it would be found by the authorities.
Later that day, after an enraged Garibaldi had been shown the body in the mortuary and told that the killer was likely a common street criminal, Ambassador Cotto had the techno-mage Finian use his expertise to "extract" Lou's final memories. Finian discovered both the identity of the killer and the secret that had cost Welch's life.
It was revealed in The Geometry of Shadows that he has a fondness for cake.
Behind the Scenes
- After portraying Lou Welch for Babylon 5's first two seasons, David L. Crowley opted to leave to show in favour of doing "other things".