From the moment they first met, Thatch displayed a strong dislike for Casper due to his friendly attitude. In "Casper's Scare School", Thatch spent most of the movie trying to get Casper in trouble and eventually succeded.
In the series, Thatch goes to shameless lenghts to get Casper punished, expelled or banished. Casper, on the other hand, usually helps Thatch out if he has a problem, although later in the series, Casper got a lot more aggresive towards Thatch and was more willing to fight back and do things that could get Thatch expelled or humilliated.
Episodes Suggesting a Conflict
Episodes Suggesting a Friendship
Thatch hugged Casper and called him "buddy", although it could be because they were in front of the Ghostly Trio.