ASSOCIATES
Bill is hardly alone in the Pokémon world. Rather, he has an entire legion of colleagues and fans to support him. These are only just the tip of the proverbial iceberg, the top few known to be directly related to Bill's career.
Professor Oak (Ookido-hakase)
Professor Oak is considered to be the foremost researcher in the Pokémon world (besides perhaps Professor Rowan), so it'd be a surprise if Bill didn't at least look up to him. Still, despite this, the only time they interacted were in Special, Electric Tale, and the anime. In the former, Bill deeply respects Professor Oak, even to the point of cringing when he's corrected by him after the Pokémon League's tournament. Meanwhile, in the anime, the respect is more mutual. Bill calls Oak the "great Professor Oak" and feels he can't turn down the man's request (to teach Ash and company about Pokémon). On the other hand, Oak also acknowledges Bill's skill, claiming that the young researcher knows more about Pokémon than even him.
In Electric Tale, their relationship is given a bit more screentime, no innuendoes intended. In "Clefairy Tale," Bill becomes a research partner to Oak on a project to study the Clefairy of Mt. Moon. Here, they seem to have a close relationship -- possibly as mentor and protégé -- as implied by their comments to each other and Ash. (For example, Oak slyly tells Ash that Bill is the mastermind behind the Storage and Retrieval System, to which Bill protests that the comment is embarrassing. Meanwhile, at the end of the chapter, Oak asks Bill if he remembers the sense of adventure Ash is experiencing, implying that they know each other on a personal instead of simply professional level.) "Ash vs. Gary," meanwhile, shows them as equals as they share a discussion about Slowpoke and, later, collaborate on studying Meowth (using Bill's inventions).
Pokémon Fan Club President
A Special-only relationship, the President is also the head of the Pokémon Council and is therefore Bill's superior. Although Bill frequently tries to help (such as, for example, bringing issues with Sabrina, Koga, and Lt. Surge to light at the beginning of the GS arc), the President largely ignores Bill, seeing him as the administrator for the Storage and Retrieval System before a competent adviser.
Lanette (Mayumi)
RSE not only introduced a new Storage and Retrieval System but also a new administrator: Lanette of Fallarbor Town. Although previous games stated that Bill was the creator of the system, a letter he sends to Lanette (viewable via a computer in her home and unsigned in the Japanese version) credits her with its co-creation. While Lanette doesn't mention Bill, he frequently talks about her, mentioning her by name in HGSS. (This is notable because he doesn't mention Bebe or anyone else involved in the maintenance of the system.) Unfortunately, Lanette doesn't exist outside of the game universe except in Special, where she holds a very vague relationship with Bill. (They're colleagues and co-creators of the system, but they don't interact that often.)
Bebe (Mizuki)
DPPt brought another administrator to canon, this time the ever-perky Bebe. Interestingly enough, Bebe holds with Bill the same kind of relationship that Bill holds with Lanette. That is, while Bill never actually mentions her himself, Bebe describes him as a close friend and colleague, both in a comment where she refers to him by name and in another one where she describes him as her "good mate in Johto." He's apparently also close enough to her to give her an Eevee, which she subsequently gives to the player character after they defeat the Elite Four and visit her in Hearthome City. Bebe has not appeared in any other form of canon except the TCG.
Amanita (Shouro)
The newest admin to the lineup with her introduction in Black and White. Because of Unova's distance from the other canon regions, she doesn't hold much of a relationship with Bill, although it can be assumed that because she's an administrator, she might have had brief contact with him at some point. It's known that she's young; that she's the sister of Fennel, a researcher interested in Pokémon dreams; and that she can be found in Striaton City, home of the first gym in the game. Other than that, she really didn't have much of an impact in the games' storyline, other than the fact that Team Plasma had managed to hack into her system without her knowing (although this was more of a side mention at the end of the games).
Brigette (Azusa)
Lanette's older sister and the co-administrator for Hoenn's storage system. Despite her connections to the system, she doesn't interact with Bill as often as Lanette. However, she does serve as a valuable informational resource to him in Special. Her only appearance beyond that was in Pokémon Box Ruby & Sapphire, a storage program linked to the Gen III games. There, she would offer assistance to the player and give away special eggs once a certain number of Pokémon have been stored. In other words, it was the direct predecessor to Pokémon Ranch.
Celio (Nishiki)
The changes made in FRLG also included the introduction of the Sevii Islands and, through a new chain of side quests, Celio. Right off the bat, Bill describes Celio as his good friend and "research buddy," but Celio quickly makes it clear that he reveres Bill as his superior. Throughout his time at the Sevii Islands, Bill works alongside Celio, praising his work while Celio responds by praising, well, Bill himself. Even after the side quests, Celio's only dialogue (which later become entries in the Fame Checker) is centered around Bill. It's implied that Bill isn't actually aware of how deep Celio's reverence for him is.
While the other programmers have more or less cameos in Special (although Brigette and Lanette offer advice that becomes vital to understanding Deoxys in the Sevii Islands arc), he's noteworthy for being more of a developed character and more of an equal to Bill than he is in the games. Here, he's still Bill's research partner, working alongside Bill to keep the man on task during the repair of the Storage System in the Sevii Islands and to assist him in his investigation of Deoxys. He's also an active supporter of the Dex Holders and once held off angry islanders to help Blue escape after she was accused of being the reason why Team Rocket was targeting the archipelago.
Brent
On Route 42 (just south of the Lake of Rage) in Crystal and HGSS is one of Bill's primary fans, Brent the Pokémaniac. Unlike a number of other fans throughout the games, Brent's phone number can be recorded in the Pokégear, after which he'll occasionally call the player to give them facts about Bill. Bill himself doesn't seem to be aware of Brent, however.