I've been looking at the Wiki and it says to unlock as Male Cosmic Hero i need a Level 100 Male Warrior, i am wondering if i can reincarnate my Male Warrior into a Male Centurion and still unlock the Male Cosmic Hero at Level 100 with him or if i have to stay as a Male Warrior until i unlock him
Unlike all future Disgaea games, it doesn't matter which Tier of the class your character is when unlocking new Tiers in Hour/Afternoon of Darkness. If the requirement is 100, any Male Warrior, regardless of Tier, that reaches level 100 will unlock that Tier. The benefit is that you don't need to reincarnate each step of the way, but if you do reincarnate, your reborn character's level still counts.
In Disgaea 2 (I believe) and onwards you would have to level the next lowest Tier to the level requirement to unlock successive Tiers. The restriction is never that you can only level up the most basic Tier.