Roger's a genetically engineered kangaroo created by Doctor Bosconovitch during his work with the Mishima Zaibatsu—basically a military experiment that actually worked out. After escaping into the wilderness, he met a female kangaroo and they had a son together, Roger Jr., who inherited the family boxing gloves tradition.
The exact origin of Roger's fighting skills is kind of a mystery, though it's pretty widely assumed he passed them down to his family. Roger himself trained under Armor King back in Tekken 2, alongside another fighter named Alex, which makes sense that he'd end up teaching his own son.
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