Character Diagonal Movement Issue

You should normalize the input vector. Like so...

 float h = Input.GetAxis("Horizontal");
 float v = Input.GetAxis("Vertical");
 Vector3 inputVector = new Vector2( h, 0, v );
 inputVector.Normalize();

Then multiply it by your desired speed and use that to move the character.
T$$anonymous$$s will correct the issue.

T$$anonymous$$nk of it like t$$anonymous$$s: Without normalization, the input vector fits into a square. The corners of a square are further from the center than the middle of the edges are. Normalizing "chops off" the excess, so the final input vector fits into a circle, making the vector a uniform length no matter what direction it's pointing.

Best,