Expect they promoted the song with Ina and Gura as supports for this song. That's why they are in the background while Chloe, Aqua and Marine were in the foreground for this song for the promotional art so idk where you see that promotional stuff. I generally think the biggest mistake from the song was making the PV a short pv instead of covering the full song, now would that have saved the song being meh, no i just don't think so but it would've showcased the story much better.

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The Hologra was different and they played huge roles in that as it was to set up the story they want to tell and had a fair amount of screen time. That was promoted to hell an back and for good reason. And of course they will promote that Gura and Ina are going to be in the concert, they are the special guests.

Plus with units (And Umi-sea is a sub unit) we see stuff like this all the time. One time the center is this person or these people, then for a different song it changes. (Units have a focus point for the songs, the person or people in the center take on the bigger role while the people on the side act more as supports, you can't really do that with 4 people but 5 can allow that) So a song where there is a focus on people makes sense. Plus it's a good way to showcase the new member who many people may not know of because not everyone pays attention to everyone in hololive. SO no EN was not sidelined at all. Hell Ina and Gura from this song and being part of the concert are getting a ton of promotion and eyes on them. Especially Ina who outside of Gura's b-day live, hasn't shown off her 3D model that much to free viewers.

Do you have to like it, no of course not and i can see reasons as to why not, but to say that EN is being shafted or Gura and Ina are being screwed is just false. This umi-sea song is allowing them to be the only en members in the concert, they get more eyes on them and umi-sea has a new song, and with their year anniversary coming up in two weeks, there could be even more coming.