Chae Seoyun

Use her early to do campaign mode, upgrading her as necessary.

You should reset her back to 1. Unfortunately, she isn’t a useful hero, and as a fighter her troop skills give bonuses to troops, not fighters. She could be used to boost the effect of vehicles, or defend the wall of your sanctuary, but there are better heroes for this purpose.

For this reason use her until she outlives her usefulness, resetting her to get back resources and use it to upgrade a better hero.