whisper is better unit than her

do some research

Kitone, Xiza, Shadowlyx and Owe are part of the dodge meta, she's much better than Whisper for sure, however you might want to read on dodge meta before jumping in, as it's expensive and requieres a lot of planning/crafting.

Where can I read up on Dodge meta and all the planning/crafting I need to do?

This is a brief guide to it
https://www.reddit.com/r/wotv_ffbe/comments/fu51ul/a_brief_guide_to_evasion_and_hit_rate/