The writers of Gust games are seldom known; I'd wager few know the names of anyone that aren't Yashichiro Takahashi, light novel author of Shakugan no Shana fame that Gust hit up to do Ryza; Akira Tsuchiya, beloved for his Ar tonelico and Surge Concerto works; and the creator of Atelier himself, Shinichi Yoshiike. And honestly, there are a lot of people, several with unclear roles.
To lazily knock off several entries at once, Yoshiike more or less handled the first five Atelier games. Whether there were others involved is somewhat unclear in my research; my Japanese is poor, but I did not notice much stand out in the credits. A couple games had an event related role listed, and I am unsure what that is. In fact, I did not learn that Yoshiike directly handled scenario from the credits; I got it from his website.
I'll slide into the games that are more relevant to most of you now: the localized Atelier games. Iris 1 starts off with some familiar faces -- Shinichi Yoshiike and Yoshito Okamura. Okamura would later, like Yoshiike, become a director, so his name is pretty notable here. Switching to the next game of the trilogy, we get a couple outsourced writers included. Of Gust, Kazushige Masuda (the director) and Yoshiike are credited, and while Yoshihiro Miyashita and Yoshijiroh Muramatsu of Dailyarts are the outsourced people.
I can already feel some people being disappointed that Gust didn't do it all in house, but outsourcing really does not speak to the quality of the writing, and there are quite a number of things to write. Main and side scenarios, NPC text, encyclopedia text... I'd say give it a chance before you knock it, especially when you learn what other games have been outsourced.
Moving on, Iris 3 has three writers. The main scenario is penned by Teruo Akaike and Kazushige Masuda (again a director), and the quests are by Toshiyuki Suzuki. The special thing about Suzuki is that he was responsible for the next four games. Yep, Mana Khemia 1 and 2, Rorona, and Totori all had the same writer. If you felt Meruru was a bit different, this would be the reason. Incidentally, NISA translated scenario as storyboard for Mana Khemia, which gives a different impression.
Meruru has three primary writers; Yasuhiro Nakai, Tamon Matsuzawa, and the director, Yoshito Okamura. Nakai is an interesting one; he's normally in game design or graphics and was director for Nights of Azure. Matsuzawa is more often credited as an event director or planning. Basically, the people who wrote Meruru are not traditionally credited as writers most of the time. Lulua was done by Yuya Jin, who did a lot of Mysterious writing.
Now we get to a series that changes writers every single game: Dusk. While Okamura was director throughout, Ayesha was partially outsourced. Within Gust, you will see Nakai as scenario direction and Matsuzawa as scene management, so it's similar key staff to last time. But we see two people from Scenario Factory Gekko as well: scenario direction by Tomohiro Takeda, and main writing by Takuya Okumura. A lot of these hands changed between games, which might explain why the direction of Dusk shifted.
Escha & Logy sees Matsuzawa and Okamura replacing Nakai as scenario direction, and Hiroki Tanaka as scenario. Shallie has Okamura, Yoshiike, and Juzo Kano as scenario direction, and outsources Decocool and Shino Ono through Pinsleep for a number of things, including plot assistance, scenario creation, and character setting. Pinsleep is also responsible for added content in the plus versions of E&L and Shallie.
For Mysterious, the writing is more consistent thanks to the work of Yuya Jin. Sophie sees scenario direction by Yoshiike, and writing by Yuya Jin and Tetsu Shirakawa. Firis shifts scenario direction to Okamura, but keeps Jin as the writer. Pinsleep returns as support (character subscenario and NPC), with Decocool, Rio Izumi, and Yoshitane Takanashi as its writers. Lydie & Suelle again sees Jin as primary writer, and Pinsleep as encyclopedia and NPC text. Finally, Sophie 2 again uses Jin alongside Genki Tomimatsu (previously on Ar tonelico/Ar nosurge).
The two Ryza games each have different writers. For the first, Yoshiike is a supervisor, Jin is support, Tsuchiya is management, and Yashichiro Takahashi (Shakugan no Shana) did scenario. A lot of people feel Ryza is somewhat different from the norm, and it is probably due to Takahashi, as the rest of the staff is very standard. Replacing Takahashi in Ryza 2 is Yoshiike, with Yuki Katsumata, Junya Tanaka, and Genki Tomimatsu as support.
What, you didn't think I was done, did you? Gust has way too many games for me to quit now, and I got side games to cover! Atelier Lise was penned by three people, one is absolutely Yoshiike, another is probably Suzuki, and I can't read the third name because my JP is scrub level. Annie is thankfully in English. Keita Sugiyama is scenario, and there are seven people credited as script writing, which I hope refers to event effects rather than writing. Lina again has Sugiyama on scenario and a bunch of "scripters" credited, similarly to Annie.
All I know of the two mobile titles is that Pinsleep is involved. They credit themselves as event scenario creation for Atelier Quest Board, and the initial character quests for Atelier Online.
For the last Atelier game I'm covering (the JP knowledge requirement goes up as I go further down this rabbit hole), Nelke is supervised by Tsuchiya and led by Shirakawa.
The rest of (modern) Gust
With Atelier out of the way, let's get to those other exciting Gust games.
Nights of Azure is penned by Miwa Shoda, and the second shifts it to Makoto Shibata as the writer and Junichi Fujisaku as scenario director. Shibata is notable for being a director of various other games, such as Fatal Frames. The shift from star crossed lovers to harem hijinks can definitely be felt in this shift, though I'd guess there are other factors at play here.
Blue Reflection's story is a bit more complicated. Mel Kishida, the artist of Arland and Blue Reflection, came up with the concept... alongside a lot of other people, including the producer, director, and writer. The lead writer was Shirakawa, who was previously on Sophie. From here, there are eight additional scenario writers, including outsourced from Crossworks and Elephante.
Second Light gets weirder, as it doesn't even directly credit a writer! If you follow credits as I do, it eventually becomes clear the only credited writer is Pinsleep, and it took them a while after release to state what they did. There were four writers; they each handled different character, date, and request scenarios. But, wait, that doesn't actually cover all of it, does it? I don't have direct access to the Famitsu article, so have this as my source, but Tsuchiya, credited as Development Producer, also wrote Second Light. Weirdly, the game credits Akiko Waba with the anime's story but cannot credit its own writers properly. Tsuchiya being responsible makes sense for me on a spiritual level, as that man doesn't miss and has done many of my favorite games, period.
Speaking of the man that doesn't miss, I left Tsuchiya's works for last because I like them the most. Everything from here on will have Tsuchiya's deep involvement in the scenario, so I'll talk about who else is credited.
Ar tonelico 1 had Akaike (Iris 3), Kei Tanaka, Tomimatsu (later went on to Sophie 2, Ryza 2), and Muramatsu of Dailyarts (Iris 2). The second game again has Tomimatsu, alongside Takemi Kishino and Seriko Tomino. The third again has Tomimatsu, alongside Saya Harada. The first light novel was by Kei Tanaka, the others by Tomimatsu.
Finally, we get to another segment where I had do do an unusual amount of research. Ar nosurge has credits in English, but I noticed something off about them. I need to talk about someone credited as Jyu-san Hanano. If I only witnessed the English credits and did not look into Ciel nosurge, I would never have noticed the issue here, and even then it's only because "Jyu-san" (13) is within the limited amount of kanji I can read. This person is also known as Juzo Kano, who later worked on Shallie. So with that in mind, ANS has Ryosuke Naya and Kano as plot advisors, with Kano as a main scenario writer. Sayaka Abe assists with Genometrics writing, Tomimatsu wrote the synthesis scripts, and Yasuhi Ohtake writing other miscellanea. Ciel nosurge credits writers of the dream world scenarios. Daishiro Tanimura (Production IG) wrote episodes 1-7 and 9-11. Kano wrote chapter 8, Izumi Ryou did 12.
I think certain fans would riot if I didn't at least mention that Tsuchiya was responsible for Augmented Reality Girls Trinary (mobile game and anime), but that's all I really know about it.
I know I just told you a million names, but it really does take that many people to keep up the writing year after year after year. For Atelier, we currently have somewhat consistent names in Yoshiike and Jin, and who knows if they'll ever let Tsuchiya out of merch storage again after Second Light. I would not take a change in writers as a sign of trouble, however. I will not speculate what goes on behind the scenes, but it appears to be difficult to maintain writers in general.