Caleb is Bernard's old 'friend' and woodcutter, who went missing in the forest a while back. You can gain access to him when you get closer to Bernard and do his personal quest.
Which is entirely possible and fine, but will probably be pretty boring when compared to the sheer scale of sexy content outside of an exclusive path. It's a game that is very pro 'exploration'. If you want a guy that's devoted to your character, and who has the most character content thus far, you can't go wrong with Bernard! So far as I'm aware, he won't touch another person besides you (and perhaps someone else, with your blessing, in future).
Actually, that's one thing I love about this game - that you can get it on with lots of different characters and most of them never make you feel bad about it, simply accepting you as a 'free spirit'. In short, generally you're not treated like a slut unless you set out to be portrayed that way, and it's nice to have the choice.
I play as a female character, and I find that really refreshing. Not just because it's SO rare to see bara-style guys who can also be romanced by a female, but also because it's nice to be able to play a female that enjoys sex as much as a man does, and not be made to feel bad about it, or cheap. I sort of see the guys as all being my big, buff, protective best-friends-with-benefits that I would probably murder for, and I dunno, it makes me feel kinda warm inside.
By the way, does anyone know if the big red lizard (Roushk?) has an intimate scene yet, beyond the kissing scene at the waterfall?