The actual prompt is "like oil and water, they shouldn't mix" but this is my interpretation going off that theme.
"Rise from the ashes and take flight, Hono no tsume!"
Karin's sword release was breathtakingly beautiful. The blade itself was fairly simple, just a little longer than your typical katana and colored a light red. The hilt had no guard and the grip was a shade of reddish-orange. At the end of the grip, a yellow ribbon extended out and loosely wrapped around her right arm and hung over her shoulder.
The most striking element of the Shikai, though, was the waves of heat radiating from it and sense of an impending blaze.
Toshiro took a deep breath. "You didn't tell me your sword was associated with fire."
"I didn't think it was relevant," Karin explained. "So, will you train me?"
Toshiro pinched the bridge of her nose. "You do realize that Hyorinmaru has power over ice, right?"
"Yeah…" Karin looked confused. "Why is that relevant?"
"We have the exact opposite types of powers. I'm sorry, but it's highly unlikely that I'll be able to help you get familiar with your abilities."
"What? Are you bailing out on me?" Karin ran up to intercept Toshiro as he started walking away.
"It would be much better for you to train with Urahara. His abilities are more similar to yours, he'll be able to help you more."
"But Urahara isn't my captian-level boyfriend!" Karin exclaimed.
Toshiro just continued walking away.
In a fit of anger, Karin swung her sword is his direction. She wasn't really thinking, just channeling her emotions into violence. But something flowed from her into her sword and escaped the tip, hurling towards Toshiro.
Karin didn't realize what she had done until it was already halfway to her boyfriend. "Toshiro! Look out!"
At Karin's warning, Toshiro's instincts went into overdrive. He felt the wall of heat coming his way, he smelt something burning behind him, and he heard the rapidly moving air reacting to the flame. In a few seconds, he had a wall of ice between him and the approaching blaze. It protected him, but melted immediately after.
Karin ran up to him. "Are you all right?"
"I'm fine, just be careful with that thing. It's a weapon after all."
Karin breathed a sigh of relief. "I'm so sorry, I don't know what happened. I was just swinging my sword in frustration and it did that on its own."
"You need to train more. You need to understand the scope of your abilities, and be able to control your power."
Karin bit her lip. "But you still won't train with me."
Toshiro cast her a sideways glance. "I really don't think that I'm the right person to help you get used to your abilities. Ice and fire don't mix."
Karin looked crestfallen.
"Although…" Toshiro reconsidered. "Since you and I have such opposing abilities, it would be more of a challenge to fight one another. If you continue to train with Urahara, I might be willing to help you gain experience in just fighting. Whenever my duties permit it, of course."
Karin ran up to him for a hug. "Thank you, Toshiro! That means a lot."
The name I used for Karin's sword is one that I plan on using in another, much longer fic where she goes through a long development of her powers and eventually gains Bankai. Hono no tsume is supposed to mean "fire claw" and her Bankai is supposed to be Fenikkusu no hono no tsume meaning "phoenix fire claw." They're both kind of a mouthful for a sword name, so if there's anyone who's more familiar with the Japanese language than I and has suggestions following a similar theme, please feel free to tell me.