The pilot did not fit with the rest of the series, particularly because it was filmed a year before the series took off in the first season. For one thing, there were some characters in the pilot that do not recur and were replaced by others: Lt. Commander Laurel Takashima was later replaced by Susan Ivanova, Dr. Benjamin Kyle was replaced by Dr. Stephen Franklin. Additionally, the Delenn we came to know and love looked very androgynous and was quite chiseled in features, as JMS intended for Delenn to be neither male nor female. This was later changed for Delenn to be female for the rest of the series. G’Kar’s looks also softened in his features at the start of the first season, as compared to his appearance in the pilot. Lyta Alexander also disappeared in the first season, but later recurred and became a series regular.
About one year before the events of the first season and one year after the Babylon 5 station became operational the new Vorlon Ambassador Kosh arrives on the station, and is promptly poisoned by an unseen assailant. The newly arrived telepath, Lyta Alexander, eventually agrees to scan Kosh, and sees an image of Commander Sinclair apparently administering the poison to Kosh.
In the council chamber, G'Kar proposes sending Sinclair to the Vorlon homeworld to stand trial. Delenn votes to abstain, G'Kar votes yes and submits a proxy vote for the Vorlons, who want Sinclair sent to them.
Dr. Kyle is eventually able to save the life of Kosh, and becomes one of the first humans to actually see a Vorlon. The crew eventually discovers that a renegade Minbari using a changeling net had disguised himself as Sinclair. Sinclair then finds and exposes the Minbari, and the Vorlons call off their attack on the station and recall their strike fleet. It turns out at the end, that G'Kar was part of the conspiracy, which had been engineered by the Minbari warrior caste in order to discredit Sinclair in retaliation for his role in the Earth-Minbari War years before.
Watch the Intro to "The Gathering" here:
Commander Jeffrey Sinclair (Michael O’Hare)
Lt. Commander Laurel Takashima (Tamlyn Tomita)
Laurel Takashima met Commander Jeffrey Sinclair while working at Mars Colony Security. She had been stationed at a corrupt mining station where it was common to go on the take in order to get a better job. Not succumbing to the culture of graft, but unable to receive a promotion as a result, she became disengaged from her work. She eventually began to get into trouble with the Earth Alliance for breaking rules. When Sinclair became her commanding officer on Mars, he agreed to seek a promotion for Takashima, if she brought her behavior in line. Sinclair asked for Takashima when he was posted to Babylon 5 in 2257, and she became Babylon 5's original first officer. However, following the assassination attempt on Ambassador Kosh, Takashima was recalled to Earth where she was assigned to a classified mission to the Rim by EarthForce. She was replaced as Executive Officer by Lieutenant Commander Susan Ivanova in 2258.
Security Chief Michael Garibaldi (Jerry Doyle)
When Sinclair was picked to command Babylon 5, he asked Garibaldi to serve as chief of security. Garibaldi became an excellent chief of security over the next few years. He managed to keep his alcoholism under control, and became well liked by the security forces that he commanded.
As one of the older members of his species, Kosh Naranek was well regarded by the Vorlon Empire. Why he was chosen to play ambassador to the lesser-species on Babylon 5 will probably never be known. The events of his arrival in 2257, however, will. Upon exiting his transport, Kosh was greeted by Commander Sinclair. Kosh exposed his "hand" in greeting. Unfortunately, only treachery returned as "Sinclair" was really a Wind Sword Minbari intent on killing the Vorlon. A Florazyne poison injected into Kosh's appendage. Dying and attended by alien physicians, Kosh was "left for dead" by the Vorlon Empire, who directed that no human should open Kosh's encounter suit. The Vorlons also wanted Commander Sinclair to be transported to the Vorlon Homeworld for trial.
Doctor Benjamin Kyle decided to risk treating Kosh, which meant opening the encounter suit. Doctor Kyle determined that without knowing where the poison entered Kosh, a cure would be impossible. Kyle convinced the newly arrived Psi Corps telepath Lyta Alexander to scan the Vorlon. Despite the very strict Corps rules in such cases, Lyta performed the scan anyway. Kosh's recovery was assured as the Minbari assassin was cornered and the telecast was broadcast through out Babylon 5 and the Vorlon fleet.
Delenn, Minbari Ambassador (Mira Furlan)
When we first meet Delenn, she is the mysterious yet apparently benevolent Minbari ambassador to Babylon 5. Her aide Lennier calls her "Satai," when he first comes aboard, indicating that she is one of the leaders of the Grey Council, but Delenn chooses to keep this information quiet. When a renegade Soul Hunter comes aboard the station, willing to murder important historical figures and steal their souls, he focuses his attention on Delenn, indicating that she will have great importance in the future. Delenn is one of the most respected and well-liked diplomats aboard the station, and the command staff trust her to help sooth tensions and conflict between different races. She quickly forms a friendship with Jeffrey Sinclair, the first commander of Babylon 5, who the Minbari had specifically requested as the station's leader. Sinclair considers her a friend and confidante, but nevertheless suspects that Delenn is withholding information from him.
G’Kar, Narn Ambassador (Andreas Katsulas)
G'Kar was Narn ambassador to Babylon 5, and member of the third circle of the Kha'Ri, the heart and the ruling body of Narn Regime. As ambassador, his hatred of Londo Mollari of the Centauri Republic was infamous. Acting on that opinion, G'Kar in 2258 was a party to various underhanded schemes intended to further the goals of his government whether they be sheltering the notorious Dilgar war criminal, Deathwalker, to attempting to incite riots on the station.
Londo Mollari, Centauri Ambassador (Peter Jurasik)
The Centauri culture, patterned on ancient Rome, is an empire in decline. It falls prey to decadence and internal politics even while it hungers to return to its days of glory. Londo reflects the society from which he comes; in the words of series creator J. Michael Straczynski, he is "overweight, prone to gambling constantly (null-pool is his favorite), and fond of women and drinks."  Considered something of a buffoon even among Centauri, Londo has been assigned the post of Centauri Ambassador to Babylon 5 to keep him out of the way.
Dr. Benjamin Kyle (Johnny Sekka)
Kyle was a xenobiologist who was chosen by Commander Jeffrey Sinclair for assignment to Babylon 5 in 2257. He was given responsibility for Babylon 5 medical department and labs. He was aboard the station when an assassination attempt was made on Ambassador Kosh. Kyle was one of very few humans to see a Vorlon first hand when he was forced to open the encounter suit of Ambassador Kosh, while working to save the Vorlon dignitary's life. Kyle was subsequently reassigned to Earth by the Earth Alliance President to work as an expert on alien physiology. Dr. Stephen Franklin was assigned to Babylon 5 to assume the medical duties. He eventually assumed the position of head of Xenobiological Research at EarthDome. When he retired in 2262, he was again replaced by Dr. Stephen Franklin.
Lyta Alexander (Patricia Tallman)
Alexander, a sixth-generation telepath, was trained by the Psi Corps. She briefly interned with the Psi Cops division before transferring to commercial work. Originally, Alexander was a P5, or a telepath of average strength. In 2257, she was assigned as Babylon 5's first commercial telepath. Soon after arriving at the station, she scanned Vorlon Ambassador Kosh, in violation of the wishes of the Vorlon government, in order to try to discover his attacker. The experience permanently changed her; she was recalled from her assignment a few weeks later and questioned regarding her experience with Kosh. Interrogated for months by the Psi Corps, she eventually escaped and joined the Mars Resistance.