Max is one of the three original chracters in the comic book, Mystery Kids (1996-1999). Max is Jack's pet dog.
Max is Jack and Bekcy's dog and has been their pet continuously since the series started. He is a brown dog with a black nose and brown eyes. In every series he has been in, from Mystery Kids to Jack & Twister Comics, viewers have always been able to read Max's thoughts.
Max got his first appearance as one of the original three characters (Jack, Becky and Max). He was draw similar to how he is seen today, but far more childish. He has always been brown and had a black nose. His main role in the series was sniffing for clues, and chasing down Willy.
When Mystery Kids ended, Max was thrown into the Jack & Twister Comics, and was a regular character. He made appearances a lot, and even had a few books focused around him. He also had a few competitions with Dog who became Jack's second dog. Max has been far more consistent of a pet though then Dog or his lizard, Lizzy. Max rarely makes appearances in the panel comics, but was seen a lot in the Picture/Text comic books.