Username: Sunnyside
Cat Name: Dapplepaw (will be Dapplethorn)
Gender: She-Cat
Age: 15 moons
Clan: Cliffclan
Rank: apprentice
Relationship: Dapplepaw joined Cliffclan when she was a mere 6-moon-old rogue, and was dubbed Dapplepaw. Not even a moon later, the clan found a stranded kittypet named Blondie, which they adopted into their ranks under the name Sweetpaw. Dapplepaw was ecstatic- another apprentice! Someone she could be friends with! Sweetpaw was... weird though. She didn't seem too excited about clan life, but she sure did like to talk. Mostly about their clanmates- specifically Buzzardcry and Rattlerfang.
Sweetpaw was assigned Honeydust as a mentor, while Dapplepaw got the deputy, Rowanflame. Dapplepaw was ready to work hard, and Rowanflame sure did make her work. Sweetpaw though... Sweetpaw didn't want to work at all. And Sweetpaw, under the guise of friendship, got Dapplepaw to do all her duties for her. And Honeydust didn't seem to care that her apprentice was the biggest slacker in the entire clan.
Dapplepaw grew to resent her 'friend'. She was always exhausted, doing the work of two cats, and only ever getting half the recognition. And one day, when Sweetpaw asked Dapplepaw to change the moss in the warriors' nests, she finally snapped. When their mentors found Dapplepaw yelling in rage, and Sweetpaw cowering, her strict mentor dragged Dapplepaw away for a scolding. Dapplepaw took the scolding, silently fuming as Honeydust coddled her apprentice. Then Rowanflame told her that he was postponing her final warrior examination until she "grew up a bit." Despite knowing it wouldn't help her case, Dapplepaw stormed away, racing out of camp before Rowanflame could call her back.
When she finally returned to camp, Rowanflame was waiting for her, his tail lashing in annoyance. "You're to help Mothwatcher remove ticks from our clanmates' pelts as a punishment." Dapplepaw opened her mouth to protest before hanging her head, and making her way to the medicine den.
That night, Otterstar called a clan meeting. After crawling out of the medicine den smelling of mouse bile, Dapplepaw saw Honeydust and Sweetpaw sitting at the front of the crowd, underneath the the Talking Ledge. Otterstar was up on the ledge, looking down over his clan. "Cats of Cliffclan, I've gathered you all here to honour Sweetpaw, as she has completed her journey as an apprentice, and is finally ready to join our ranks as a full warrior in her turn." Dapplepaw struggled to keep her fur from bristling and her claws sheathed. This was so unfair! Sweetpaw hadn't done any work her entire apprenticeship, Dapplepaw had done it all! And yet, Sweetpaw was getting her warrior name first. Dapplepaw narrowed her eyes at the newly named Sweetbriar, and made a vow to herself to undermine the new warrior in whatever way she could.
tl;dr - started as friends, then Sweetbriar abused their friendship and they're now nemeses.