It's believed that Balzax is named after Ed Boon's cat, who once attacked John Tobias and gave him lyme disease. Balzax has been around for many ages, but did not actually do battle himself until MKII. Once you've unlocked Balzax, all the character endings will change to their true nature. Often it's because Balzax had a part in the character's story. After MK1, Balzax had lost contact with John, who had been captured by Shao Kahn again. Luckily, Balzax met up with John just after he had defeated Kahn, and started a psychological treatment to free him from his split personality.
The Shocking and Compelling Lifestory of Balzax, Cat of Death by FatSatan
Behind The Mop
Balzax was originally made up by FatSatan, and developed as he and McDaniel joked about Balzax' secrets in various MK games. Brandogg then started making fake picture of Balzax, after which he officially became not a regular talking cat, but a giant talking cat.
Balzax' MK2 stance
Balzax' MK2 Eatality
Balzax' MK2 Claw Slice
Balzax vs. John
Balzax' MK2 Bio
Balzax' MK2 ending
Scorpion and Sindel's endings in Balzax mode
In both MKII and MKT, there was a special mode of gameplay that could be activated by unlocking Balzax. In this Balzax mode, the endings of the characters would change due to their run-ins with Balzax.
Balzax in MK4
Balzax' MK5 bio
Announcer sound clip
Check out Balzax in:
Mortal Kombat Mopologies: John
Mortal Kombat Mopologies II: the Johnicles
Mortal Kombat Mopologies III: Balzax
Mortal Kombat Mopologies IV: Deadly Appliance
Mortal Kombat 4.5
Mortal Kombat Platinum
Death From Above