Somebody please explain it to me! I thought it was a ring at first.. but now I'm confused.. why a key?
Shuna had the key since before, when he came to Kanchan's apartment he had it in his pocket, there's a panel showing that.
Whether it was his own or he wanted to give that particular copy to Kanchan, it's not clear.
You are on the right track. If you read enough of these you learn that giving a key to your lover is a big deal in Japan. It's a big next step because it means you trust them to come in and out as you please, and often because it suggests you want them to come over whenever they can. A key is something characters have fought for in other mangas (usually a tsundere or older uke doesn't want to give the overly affectionate seme a key because he'd never leave and/or want sex all the time and the uke is protecting his @$$). For this couple giving a key would be a huge deal -- like one step below an engagement ring.
I don't remember her at all Σ(っ°Д °;)っ
did she appear on any previous chapters?