I've writing a fiction with a crazy MC and his crazy friends, but I want to introduce a "good" character in my story.
He would be the one stopping them when they do things too stupids, but I think that I've been describing tooo much the MC, and now I can't describe this new guy without making him bad ass.

How about a Character that is too good, that the MC is irritated by him. MC decides he wants to prove that there is no person that can be THAT good. and so he started "bullying" and doing all sort of things to the goody-char. Yet goody - keeps forgiving him.
Goody doesnt have to be someone who is weaker than MC. But he can pretend to be one infront of MC while in secret he is super strong, and MC finds that out . Basically MC discovered that goody wasn't forgiving him because goody was afraid or idiot, infact had goody not forgiven him- Mc would've had been beaten up.
well that is what i was able to comeup with on the spot, idk how much it would be useful for you .

The easiest way is making that new guy mysterious. So you dont need to describe him, rather you let readers describe him. I mean, instead of telling readers his backstory and his personality, just make it no one know anything about him besides his name. And every time the mc trying to do something crazy this character will try everything to stop him. Whether He succeed in stopping the mc is depend on you.

You could always cut back on the descriptions. Let the Point Of View, what the characters do, and what the characters say, describe him for you. Best to try to show, and not to tell. I have an example:

MC: Hey, that's a nice car! Should we take it?
CrazyfriendA: Yeah. We can pick up some chicks too!
CrazyfriendB: I call shotgun!

Sane Friend: Guys, you know how long you'll be in prison for this? I'd rather go see the new boxing gym. It's just down the street from here.

CrazyfriendA: Awe man, it's dweeb genius.
CrazyfriendB: Glasses, necktie, and all.
MC: Shut it you two! He's my main bro. What's up, Jeff?