I have found the Soldier and Engineer but I can't let Engineer come with me, how do I get him and how do I unlock other classes?

hello i just got the engineer, what you need to do i think is get the heavy and there's a secret room where you can get a skin of the sandvich, then you can get the engi

maybe is just the brass beast with some chips

the heavy is in the map where you fight the sniper boss close to the end of the level