Kerillian, or "Irish bow girl" is a character often played by people who think their character's pointed ears make them invincible to Special attacks and thusly wander far away from the team. Even I've been guilty of it, and I'm a Vermintide 1 veteran with a couple hundred hours. Why is this? No one knows. I don't know. Do you know? I don't. One'd assume it'd be because Kerillian is mostly played by ♥♥♥♥♥♥♥ who think elves are the best or horny teenagers who immediately gravitate to the only "Attractive" female character. Or weeabooos. A suprising amount of weeaboos play Kerillian. No, I don't know why, either. And no, I can't answer the question of why weeaboos suck at every game they play, that's one that haunts my sleep at night, too.

Regardless. The obvious problem is that Kerillian players either tend to suck ass or have the spacial and teammate awareness of a blind, drunk, and deaf lumberfoot.

There's no way you can stop your ♥♥♥♥♥♥ Kerillian from being ♥♥♥♥♥♥. Sorry, that's just how life works. However, you can stop the one ♥♥♥♥♥♥ Kerillian (Or any other character) from ruining the entire match on their own.

Step 1: Keep them away from Grimoires.
For obvious reasons. Never trust a player of questionable skill with grimoire locations.

Step 2: Keep yourself and your teammates away from Grimoires.
Grimoires require the team to all be on the ball, even more so than in Vermintide 1. If you see any of your teammates acting stupid, know that with one grimoire that becomes worrying, and with two that becomes a death sentence for the whole team.

Step 3: Always keep a healing kit on hand. If that fails, keep a potion on hand.
This one should also be obvious. A ♥♥♥♥♥♥ player tends to get hurt often, but if nothing else, keeping them alive keeps one more player around as a meatshield if nothing else.

Step 4: If they'll listen, try giving them polite tips. If they won't listen, feel free to unleash your salt on them, it'll make you feel better if nothing else.
There's two kind of ♥♥♥♥♥♥ Kerillians and indeed, ♥♥♥♥♥♥ players in general. There's the n00b, who's ♥♥♥♥ because they just picked up the game and are still learning. Be kind to the n00b. They don't know any better and they're likely just as frustrated with their own ineptitude as you are. Give them polite pointers, and if they snap at you, just gently herd them along and offer in-game support. Then there's the ♥♥♥♥♥♥♥. The ♥♥♥♥♥♥♥ doesn't care that he's ♥♥♥♥, in fact, he probably doesn't even realize it. He probably thinks he's great. ♥♥♥♥ him. You'll be able to tell this type if he gets salty at the team after going down or just ignores the team period.

Step 5: Watch their back.
Even if they won't watch yours, they're not doing any good if they're down halfway across the map. Everyone should be in groups of two at the least. So if you're coordinating with the team, the best individual player should stick with them. Alternately, the tankiest.

Don't get mad and be patient if they're new. Keep heals around if you can. Watch their back, even if they won't watch yours. And for the love of god don't grab grims.