Tinkers believe themselves to be experts in the fields of magico-mechanical engineering, firearms, and alchemy. In fact, their knowledge is incomplete, often dangerously so, and only the lucky few survive long enough to produce technologies that consistently benefit their users. Like many thinking people, they tend to be physical cowards. They make poor front-line fighters, but their grenades and guns can help win battles.
Defense and BAB progression are very similar to the SRD rogue's. Tinkers can't use shields or wear mundane armor with greater than +3 AC, and they are limited to using ranged weapons and, in melee, only simple blades. Tinkers have average HP and SP (in Iceblink, SP power spells and spell-like abilities, of which the tinker has several).
Design-wise, this is the class closest to the traditional rogue, with less emphasis on stealth/sneaky abilities, which I didn't want to bother with (I have reasons for this I'll get into later). But the rogue is only one of two classes with access to Engineering, a trait that functions in much the same way as trap detection skills in other RPGs but has some broader applications too.