Hi!! Thank you so much :) I'm glad you've enjoyed it so much!
Yes, Hallie is the only "datable" npc, I didn't write anything for Katey; that might change in a future update with how many people seem to really want to see that.
So in-game its actually called the Bail Ending. You get it by first "confessing" in her dream, then rejecting the offer for Hallie to replenish Sam's drink, and also triggering a flag at one of two places (and if you triggered it before, you can potentially undo it at the second juncture): Select "Answer her directly...?" triggers the flag, and "Its nothing special. Katey's mom can do it better." You can select any other option before, but you need to select "Its nothing special," otherwise "Its nothing, but yeah, I can do that." undoes the flag. You'll be given the option to "Confess," but Sam winds up not doing so.
So to give a full rundown:
Game Start > You're beautiful. > No > Free choice > Free choice > "It's nothing special. Katey's mom can do it better" > Free choice > Look around. > Stick it out > Confess
Also, I literally just found a programming mistake that doesn't put the correct Katey sprite for the last line of dialogue... so before you go and follow my instructions for that last ending, let me go fix this and upload a new update, haha. Version 0.93 should have the fix in the next half hour.