Katie is your friend and... ...uh... a bit of a... masochist...

but for now it's chill- she's just invited you over to her place to hang out, and you're there and relaxing and having a good time-

"Mmh- I'm tired.. ...but not too tired to do THIS-" She tickles you like mad until you lose it and finally, though, has mercy and stops- "Heh, silly."