This feels like a pretty good prototype. It does a good job of showing off the the mechanics and art, both of which are pretty good. The five idols in the game so far are distinctive and have interesting, and the pixelart used to bring them to life is great.

But there are some factors that make it feel more like a prototype than a demo, if that makes sense. The story scenes, while showcasing how cutscenes will work in the final game (including, again, excellent pixelart), tell a pretty barebones story which mostly consists of partially explaining how the Hololive girls ran into each other and why they're fighting slimes and goblins. (Their pixelart is also good.)
Speaking of which, the combat is really easy there's a lot of potential depth in the mechanics, but rarely much incentive to use it. I imagine this is something you're planning to tweak once the game is more complete, so I won't spend much time on it.

It's easy to settle into a specific strategy for each character that efficiently cuts down the goblins and slimes faced. I imagine that more difficult encounters might change this; focusing fire on a specific enemy might be more enticing than dealing a bit more damage spread out over a group, healing might be more important, etc. Three skills stick out to me as particularly use-impaired:

• Flare's Firecracker—Moves her backwards (losing access to her more potent offensive skills) and doesn't deal damage. The 25% chance of stun sounds nice, but Noel does that with every attack. Oh, and she sets the enemy ablaze, but all her attacks do that, don't they? Either I don't get how her passive Ablaze ability works or this just doesn't do much.
• Marine's Spark—She can get +30% dubloons next turn. You know what would get three times as many dubloons? Attacking! I like the idea of the skill, but it's outperformed by having Marine attack twice. Maybe it's useful if you're planning to use a high-damage attack that you can't use every turn, but that's just her Money Shot, which doesn't seem to give Doubloons. (And usually ends combat anyways.)
• Pekora's Carrot Toss—Her Peko-Cannon deals way more damage when it uses only one carrot. It's only useful if you have a lot of carrots and want to actively conserve carrots...which is unlikely, given how short battles are, and also why not just forage?

To end on a positive note, I've looked at the YouTube channel and I like the UI changes I've seen. It looks like the game's going in a good direction, and I look forward to seeing how far it goes.