The big problem with the Kafers is that they're something of a hot mess as far as what they are and are practically unplayable in the state they were originally written.
I'm not sure how they're detailed in the latest version of 2300. I know that Colin had some ideas. I have my own extensive interpretations, but I'm going to stick with canon as of the ancient
Kafer Sourcebook that came out for the original 2300 game, back when ... well I suppose when I rode a brontosaur to the game shop.
Sangelet said:
I have not been able to find anywhere information of armor that kaefers can wear in combat.
The only canon armored item is a Kafer environment suit that is listed as having a Armor Value of 0.5, which is better than human hostile environment suit. Otherwise, they apparently wear no armor. The implied reason is that Kafers are essentially masochists. Their intelligence and pleasure are both tied to pain and danger, so they don't really do things like wear armor or have painkillers, even. Obviously this fails under even the slightest examination.
Sangelet said:
I know that kaefers can be treated as cannon fodder in some adventures
This seems to be a popular interpretation for them, despite the fact that in the Kafer Sourcebook and other places, Kafers are repeatedly described as consummate warriors that are basically like some combination between Captain America and Hannibal Lecter once they become intelligent. But most writers for the game seem to prefer "stupid aliens that use human wave attacks to make humans feel heroic" (as suggested in the art in the Invasion sourcebook, for instance).
Sangelet said:
And I think that some pictures seem to represent kaefers as wearing a sort of helmet although I might be wrong and this just represent their skull bones.
It is a helmet, but it's essentially ceremonial item with no armor value assigned to it. It is thought to be a badge of rank; Kafers only occasionally wear it in the field. (You have to wonder why Kafers, a species each of which are like Carlos Hathcock the sniper, would ever have any kind of visible rank tags on themselves, especially since officers are supposed to be the smarter Kafers and Kafers, once intelligent, are stressed to have a nearly superhuman ability to pick out human officers and snipe them even when officers have no visually distinguishing signs in their uniforms or equipment.)