Robin introduction?

If I remember correctly, he would just show up. I think the idea or some dialogue pointed out that he was in college so he would show up and help on breaks and stuff. However, there is an amazing episode (maybe a 2 parter) where it shows Robin's back story. It was just aired after Robin had already been on the scene.

When I watched Season 1 for the first time about 12 years ago, I noticed there weren't very many episodes with Robin. He got more episodes in season 2.

A bit. There's an episode were you see Robin's origin story being told in flashback.

By production order (when BTAS was first broadcast it wasn't shown in production order) Robin first appeared in episode 2, Christmas with the Joker, first shown November 13, 1992. He wasn't in that many episodes of the first season, but his origin was shown in flashback in the two-part Robin's Reckoning, first shown February 7/February 14, 1993 (episodes 32/33). IIRC there were references once in a while to his being at school for much of the time.