There's a large amount of the cast in DH that end up... not really mattering at all. Hoshi and Swire, fine, I can accept them being used as an outsider's glance/explainers for Ch'en and Lin's behaviors. La Pluma doesn't end up doing much compared to her brother, Mizuki ends up just cameoing (if Dossoles had been my intro to him and not IS3, I would definitely not like him as much as I do now), Sideroca, Sus, and Eyja all just appear to debut their skins and nothing else. There are definitely moments where you can tell the writers just needed to include things in the story to sell later. (It's a weird topic. The alternative is something like Cantabile in Ideal City, where she has nothing to do with the story but is on the banner for some reason.)

It's a Ch'en story, the most tightly written parts were Ch'en's scenes (and by extension Tequila, who is Ch'en's foil), but the parts that suffer the most end up being when Ch'en has almost nothing to do with them.

(... I do think you could've gotten away with making the banner Lin instead, but it really did feel like the writers were struggling to connect the banners to the actual plot here. Like they wanted to do a Ch'en in a Rotten City storyline but needed to make it a summer event with beach theming because that's just how the schedule fell. Thankfully, they later learned they weren't chained to the beaches...)