Here are some links to explanations about the characters and ending of Infinite Jest. CAUTION: These are all spoilers and should not be read or analyzed before you, the reader, has read and put some thought into Infinite Jest.
What happened at the end of Infinite Jest? With the help of a few others, Aaron Swartz thinks he has a compelling answer that can be found here: Infinite Jest ending explained
Wondering what happened to some of your favorite characters in Infinite Jest? Michael Monts has some interesting ideas that can be found here: All character endings
As theories go, I.D. has some very valid ideas of what happened to the characters, whic can be found here Blog of Explanations
Lastly, the ultimate guide to everything Infinite Jest can be found here, and is sure to answer all of your questions! (maybe). This link to links details Mario, Joelle, and many more interesting characters, as well as showing you even more interesting theories and explanations.