Kal is jealous of Jason because Neri cares more for Jason than for him. From Season 3, Episode 14:
Kal: I don’t want to know about Jason.
Brett: What’s wrong?
worries about him.
Brett: And your point is?
Kal: She likes him, more
than me. I feel… Bad. Do not understand.
Brett: Don’t be such a wuss,
they’re just friends. Hey, here he comes now.
Kal: Neri will be with
Brett: Yeah sure, she always is. Hey, ease up. People will think
This may be because Kal sees her as his only friend, and does not want to share her with anyone else. This is especially true in light of Neri coming from the same planet as Kal.
From Season 3, Episode 17:
Brett: Come on, Kal. You can’t just stay down here forever. You’ve got
to get your head around it. Neri’s not just your friend. She’s
Jason’s, too, and mine, and…
Kal: Neri… My friend.
Brett: Yeah, but you can’t just own someone, Kal, and you don’t just
have one friend, I’ve got heaps.
Kal: Not me.
It's unclear whether Kal has a romantic interest in Neri, or is merely lonely.