I'm trying to set up a Joomla 2.5 site w/ K2.

I added a test item, but in MySQL, the entire content of the item goes into the "introtext" field of ****_k2_items while the "fulltext" field is left empty.

I imported many records into the database, but the fulltext becomes empty whenever I edit the item.

I even manually moved the contents from introtext to fulltext, but the same thing. After I make any change to the item, content goes to introtext while fulltext is empty.

I'm not yet planning to use Joomla 3 because some of the extentions that I want to use are not compatible.

You are not manually specifying an introtext.
You can do this either by using your editor's read more button or using two separate editors, one for full text and one for introtext.

The latter option is located under the component's params.

Thank you. I guess I misunderstood how it works.