If its a second MC, then I tend to introduce the idea of them at the beginning then move onto the other MC. So in the book I'm working on now, the second MC is at the very start of the book, but you don't get his name or looks, just that his in trouble and running from the police. Other characters I tend to bring in through the MC. So the MC could say,
"I've gotta get my bike fixed. We won't get far with the tank leaking. Apollo should be able to do the job."
By mentioning the character through the MC, it's more easy to the reader. You see what I'm saying?