Kyouko is intended to be able to set up and reflect attacks.
Her Reflection feature is the main point of her fighting style. If she's paired up with an ally that can deal thunder damage, she can use that to focus down a single target of her choice. Her own Yell action also allows her to set this up.
Example 1: Kyouko is fighting a fighter, a monk and a wizard. She uses Yell. The fighter and the monk are both within 30 ft. of her, so they make saving throws. The fighter passes, while the monk fails. Kyouko rolls a total of 12 damage; the monk takes 12 damage, while the fighter takes 6. However, Kyouko uses her Reflection feature to cause the wizard, who is 40 ft. away from her, to take the damage. He takes a total of 18 damage, while the fighter and monk are unharmed. Since they took no damage, they then deal no damage back to Kyouko.
If she waits to use Reflection, she can still deal damage to the fighter and the monk, while still doing a lot to the wizard.
Example 2: Kyouko is in the same situation as above. However, this time, she does not use Reflection, and the fighter takes 6 damage, while the monk takes 12. The fighter then deals 6 damage back to Kyouko, while the monk deals 12. Kyouko then Reflects this damage to the poor wizard, who takes 18 damage.
She can also Yell off her allies for extra firepower.
Example 3: Kyouko and two others are fighting a monk. She uses Yell, causing her two allies to make saving throws. They both fail, and Kyouko rolls 9 damage against each of them. She then uses Reflection to prevent them from taking any damage, and causes the unfortunate fighter to take 18 damage instead. Her allies, who took no damage, deal no damage back to Kyouko.
So, Kyouko does her best in a crowded fight. The more people within Yell and Reflection range, whether friend or foe, the more dangerous she becomes.