Let's get right down to it, starting with origin of their personalities and design.
Star Guardian Ahri and Janna are based primarily on Sailor Venus and Jupiter respectively.
In episode 19 of Sailor Moon R, both Venus and Jupiter constantly tease and stab at each other with remarks about how they keep holding on to the memories of their old boyfriends instead of finding a new one. This ultimately causes beef between the two until they both have to fight off _creatures from an alternate dimension_ with each other, acting friendly but deep down still ticked off.
"Love will tear us apart... If the creatures from an alternate dimension don't first."
Now, again. This is only half of what's missing between the relations of Janna and Ahri.
"It's not the big lies that tear us apart, Janna! It's the little secrets."
This is still unexplained, but soon enough I shall create a theory on that as well.
"Falling in love. That's how a star falls."
"All that sparkle and still no personality."
"Loving too deeply. That's how a star falls."
This is what gave it away.
Star Guardian lore wise; Ahri and Janna are indeed veterans of both teams. Meaning they've known each other much longer than Ahri knew Miss Fortune and the others.
Hopping back to Sailor Moon as reference, Ahri's phrase "All that sparkle and still no personality." would most likely be an insult to her bad reputation with guys.
Now, since Janna isn't a Legendary skin; she has nothing to say to Ahri obviously, but that would be exactly like what she'd do anyways.
Along in Janna's defense, Star Guardian skins were color-coded to represent which lane they're in.
Pink = Mid-Lane
Blue = Jungle
Red / Orange = Bot-Lane
Green = Support
Purple = Top-Lane
Again, Ahri's team existed longer than Lux's team and so did Janna.
But why would Janna, a pure support; be laned against Syndra, a burst mage?
Janna was most likely secluded from the others into top lane because of Ahri's jealously and slight rivalry with Janna.
Janna, because of how quiet she is simply said nothing and got used to standing up for herself and fighting for herself.
Now, Ezreal hops into each lane time to time; being the only available guy to hit on.
Ahri, just like Sailor Venus; wouldn't try to date him because of her pride. She's too prideful to try to date him.
Janna, just like Sailor Jupiter; would try to date him with every chance she gets. ( She's a support, of course she'd try to hit it up with an ADC. )
However, she'd of course act emotionless and be busy studying ( Burning Bright Video. ) than to be chatting with Ezreal.
Ahri would play hard-to-get and act more strict around him to stop herself from wanting to date him.
"I guess you couldn't hang with the cool kids!"
What do you think? Why do Janna and Ahri hate each other so much?
And who can explain why she says; "It's not the big lies that tear us apart, Janna! It's the little secrets?"
And what skin lore theory should I do next?