This guy shows up in Hythloday's story about his last visit to England. And yes, he is yet another actual, historical figure, a former Chancellor to Henry VIII (the same king More works for). Kind of like dear old Giles, the Cardinal is an example of someone exceptionally friendly, generous, and fair. Although Hythloday is critiquing political positions very similar to the one the Cardinal holds, he acknowledges that Morton is a good guy, reminding us readers that not every criticism Hythloday makes seems 100% fair.
Morton is all about facilitating fair and insightful conversation, something obviously central to this book which is pretty much one, long conversation. As a perfect host, Morton brings together an interesting group of people for a dinner party, allows conflicting points of view of come up, but then steps in when he think someone goes way out of line. If you've ever been to an awkward dinner party, you can appreciate these kind of skills. Check out his expert handling of the know-it-all lawyer:
"Hold your tongue, for you won't be finished in a few words if this is the way you start. We will spare you the trouble of answering now, and reserve the pleasure of your reply til our next meeting." (1.21)
These skills would also have been particularly important for a time when long dinner conversation was a primary type of entertainment. Remember, we're in a no TV/Internet/Wii universe here, so, believe it or not, debating ethics over some roasted boar was considered a pretty great time.
Morton is also a useful way for More to cover his back. As you may have picked up, there's some pretty controversial stuff in this Utopia book, particularly toward the church, and More would not have wanted anyone to think he was actually against the church. So look, see here, even a cardinal is down with what Hythloday is saying. Phew.