I would like help making up a class

[waffletaco]

I've been having some ideas bouncing around my head for some time (a day or so) and I think it would be a fun RP-ing class. It's a class that is an npc class normally (commoner or expert or choice through RP like a nutty aristocrat that saves himself from assassinations) but every round in combat, you get a chance to roll to become a fighter of one level higher than the amount of levels you have taken in this class(in terms of hit dice and feats). However, I've never played D&D--other than computer games. I think I have a decent grasp of the rules, but with suggestions from the fine players of this forum, I think this class could really turn into a fun and useful one.
The RP-ing aspect of this class is basically the use of split-personalities. This class gets a +2 to sense motive and some other skills that stack as one levels in this class (via class feat)
What should the roll be to access the Fighter personality? I'm afraid I don't know a lot about probability, and I don't want the roll to be too easy or too hard--a 13 maybe?
I'm sorta sleepy right now. I would like some feedback and help. A name for the class?(whether or not I choose yours, it really helps me think up of one as well because I'm bad with words without that extra nudge) What feats would this class get as he would level up?--choose from any handbook if it seems relevant to the class or even make up your own.(this is homebrew after all)

Sorry this is all bundled up and messy.

[DracoDei]

This seems like a one-off character concept, rather than one that would have a noticable number of members. Which isn't to say one couldn't figure out the level progression anyway...
The questions of how often re-tries are allowed to switch into the battle personality and how long it lasts once you do, and what the drawbacks (if any) are come to mind.

[Xefas]

I agree with Draco that your idea is less of a true "class". However, you may want to look at the Giant's Champion Class (http://www.giantitp.com/articles/mYkD5jL8N9SAcClN3pZ.html), as it represents a similar multi-personality kind of thing; transforming from weak squire-boy into powerhouse of doom.