OK... but why do i have to play with my Resistance character? ... also...
ehmm also i dont get why... in the Story mode of the Security.. there are under the Security missions again the Resistance missions.. but they display again an random Resistance guy XD
You don't always get to play Security in multiplayer, just like you don't always play as the COG in Gears for example, you sometimes play as the Locust. In Brink you play as both Security and Resistance so you have a character for both sides that you customise, if you want to of course.
There are two sides to the campaign so you have the option of playing out both, the Security missions are at the top because you chose to save the Ark, had you chosen to escape then the Resistance would be at the top, it doesn't matter what order you play them in or whether you play the Resistance side at all, that's your choice.
Hope that clears it up for you.