This is missable, and a more than a little annoying. On day 5, you'll be offered the option to romance one of the others.
SAVE HERE.
You can only romance one in a playthrough, and it doesn't pop until the end of day 8. In order to romance one of them, their meter needs to be at the top when you look at the stat screen (
). (The one exception is Juniper.) Afterwards, you have to play through the rest of the game for it to pop. If you have more than one person at the top level, you can reload your save from day 5 and replay the end. (You can do the Juniper and No One paths as well.)
Furthermore, in order to romance Gavin, you MUST have the Blue Scales icon at or near the top, or else he will reject you.
You're going to have to back up further in order to adjust the character levels to have the chance to unlock them. You can try to romance them, but if they reject you, you'll have to reload from an earlier save to try to get their stat up.
Hope this helps!