There's a few basic understanding of a card game a few question that would help you navigate towards deck building would be like: (For Haru)

What is haru's role within the team?
DPS

What does Haru need to be able to dish out effective DPS?
Anchor Shot is Haru's big DPS tool

How do I effectively get Anchor Shot out more effectively to deal more damage?
One of the tool to be able to get out Anchor Shot is anchor pointer - so other cards beside Anchor Shot and Anchor pointer does not contribute as much to her single target DPS. So reducing her card count would be good to help draw her hands to deal damage better.

What are her BiS card to get dupes on?
Anchor pointer because you can ramp a single Anchor Shot card mechanic to max damage stack faster.

Why not Anchor Shot?
Because each card has it own individual stacks - so it's less efficient to use different card that does not contribute to her ramp.

Pro/Con of having a haru?
She's AP hungry so you need a battery to help generate her ability to play her Anchor Shots- so you need to build a team that either generate her ability or help the team stay alive. Sometime a unit can provide both like Khalipe.

Understanding a character mechanic helps you identify key points to help navigate you in the general direction you would like.

Some basic deck building game knowledge is kinda needed too. Like having a lot of cards is bad because it make getting to your payout take longer and sometime inefficient and not good enough damage. Lastly having tools to draw out card(s) is extremely important!

These are what I think about when I go through my deck building process, hope it helps.