I (my f2p husband) has a 3.27 Valen and just pulled Krill. He is keeping Valen since he has invested so much time into him.
Should he keep Krill?
Kiril the 4* or Karil the 3*? I’m guessing Karil the 3* based on the tags you used.
I don’t see any harm in keeping Karil if your husband has roster space and is sort-of new to the game. Karil has a decent attack stat and hits three, could be useful for color-stacking in the early game.
Yes Karil the 3*. Thank you. I only had an auditory explanation of his name.
He likes to condense down to 1 of each color but is starting to learn about monochromatic teams. In turn, he is starting to ask more of who to keep and who to feed.
yes, you’ll join an Alliance and also aim to participate in war. You’ll be glad to have him instead of keeping 2s and 1s to use all war flags 
No need to feed any maxed 3* or higher unless you are running out of space.
Using two or more of the same colour heroes increases your damage massively. If, for example, you’re doing those quests with all red mobs, or mostly-red map stages, two or more blues in the same team will really knock them down quickly. By comparison, a rainbow team feels like hitting them with a sponge.
Personally I like to stack 3-2, so at least three of each colour keeps your options open.
Yeah finish valen then start on karil also remember to fire valen first then karil as valen defence down will be better used.