Kristijan the Hedgehog was actually Kristijan Matijević himself, only he was transformed by Dr.Robotnik into a green Mobian hedgehog. He's as fast as Sonic, but not that fast as himself. After being transformed into a Mobian, Kristijan wasn't feeling quite alright about it. He goes to a mission to bring himself back to normal, and he does at the end, but his Mobian version of himself is featured in two new series.
Kristijan the Hedgehog as he looks like in The Mysterious Mobian Secrets
Kristijan the Hedgehog as he appears in The Good, The Bad and The Mobian