For Resonance all you need to level up the heroes you put here is their Stars not their level. They are getting leveled up automatically to the min level of the top 6 high level heroes shown in the top right area. So if your top 6 heroes are lvl 310+ the heroes in resonance will be leveled to their maximum by star level allowed, or the minimum of the maxed leveled heroes. red stars=360max, gold stars=310max, white stars=260max, bronze stars=210max, yellow stars=160max, silver stars=110max, etc..
The resonance heroes are being leveled automatically you do not spend gold and power points on them, only star evolving.

Ah, now I know.. the girls will get temporary:

- the max level their own star allow, OR
- the resonance level

That what is lower. Found that in another guide. And that's why they cannot be levelled manually as long as they are in resonance.

Do I see it right? At your resonance picture.. is the minimum level for the girls.. level 110. So I need to level up my girls to level 110?!

( my own resonance is level 100 but the girls are still level 60, each of them has a "max level" infotext at their avatar )

But you wrote about the resonance.. we should use girls we don't plan to level up.. but level 100 or level 110 is also somehow expensive.. for me.. so far.