Very cool demo with some fantastic art design, interesting worldbuilding and an overall level of polish beyond the usual indie projects I see posted here. This feels like a big budget release title with a lot of time and love put into it and I'm very interested to see how the game develops. If you're here just wondering if this demo is worth playing through or not, then I'd say if you like games like Transistor or Hades then absolutely, Brightseeker is worth the download and the time to try it out. It's a free demo and a very well put together one at that.
Since this game is still in alpha and will probably undergo a lot of tweaks, I'm going to give some specific feedback based on my play experience:
It's difficult for me to adjust to the isometric movement of the arrow keys right away, but I started to get used to it near the end of the demo, so I imagine it will grow on me, though I think players used to the controls in other isometric action games will get frustrated. Or maybe the game was mainly designed with controllers in mind? Either way, I tried adjusting the "Isometric North" setting in the options and it didn't have any effect. Once this option is functional I think it should be turned off by default and toggleable for people who prefer the current movement.
Also, the demo really encourages players to spam the dash button constantly while poking away at enemies from across the screen with projectiles. It's way easier than to try to engage enemies in melee (which inevitably results in the player taking damage) or using the extremely tight shield parry window (which feels nearly impossible to get deflected shots to actually hit enemies). It doesn't help that enemies in the demo are incapable of catching up to a backdashing player and have no countermeasures to prevent this. There really aught to be a cooldown on the dash and ways to make using the shield more rewarding, this would encourage the player to constantly mix up their gameplay. I'd like to see some melee combos too to add more finesse in how attacks are executed.