>>538060080
It's not just her cost recovery it's also her yellow buff and her shield.
But yes, the cost recovery does add up as you use it more and more. If you do have the extra ligma to spare then sure, you can go ahead, Seia can be basically slotted into any raid so it'll never really be a waste.

>>535932687
Yes, but also no.
Seia's EX applies to EX skills, dHina's 0 cost shots are EX skills so they'll waste a usage. However, Seia has cost recovery built into her EX, which will give 1-1.5 cost bar depending on the skill level, UE level and cost recovery special.
>nyFuuka wastes 2 cost to halve dHina's 6 cost EX into a 3 cost skill => you save 1 cost bar vs using her without her
>Seia wastes 3 cost to halve dHina's 6 cost EX into a 3 cost skill + you get 1-1.5 back => you save 1-1.5 cost bars vs using her without it
so she's better cost wise, but nyFuuka's advantage here comes from her crit damage, though you'd be better off just using Rio+dAru instead

>>532171665
Seia recovers cost with her EX buff, she also halves the target student's EX twice at level 5. So as long as you're using it on a skill that costs 2+, you're gaining cost, all while cycling back to your DPS.

>>530531565
It's equivalent to having another student in the team.
Here is the exact cost she gives based on whether UE40 or not and/or with cost recovery specials.