In episode 12, when Ken and Mamoru are in the helicopter with Crashers, Mamoru calls Rex on his cell phone and tells her that he and Aya are okay but Ken is badly injured and Yohji is missing. But in episode 13, Rex and Mamoru are alone in a car when Rex tells Mamoru that no one has heard from Weiss since their last mission so it's probably safe to assume that they're dead. Sure, it might be safe to assume that Yohji's dead, but Rex should know that Ran and Ken are still alive (especially Ken--she made his hospital arrangements!). Judging by Mamoru's later conversation with Knight, Kritiker's official position on the matter is that the members of Weiss are all dead, but Mamoru knows that at least two of them are alive, and Rex knows he knows, so why would she pretend, in a private conversation, that they're not?
The only thing I can possibly think of to explain this away is that Rex and Mamoru may be speaking of another Weiss besides Aya and the boys, one we never see on screen. Knight says it's the "fourth-generation Weiss" which has been sacrificed, and this is confirmed when Rex asks Mamoru for his opinion on recruits for the fifth generation.
Given that Aya, Yohji, Omi, and Ken were definitely the third generation, there are several possibilities for the identity of the fourth generation which has gone missing-presumed-dead:
1. The loss of Omi and addition of Sena and Kyou changed the third generation into the fourth, making Aya, Ken, and Yohji members of both. This is the position I tend to adhere to. However, if this is the case, as I said before, Rex knows that two of them are alive.
2. Sena and Kyou constituted the fourth generation all by themselves. If this is true, then there should be no doubt in anyone's mind that they're dead. Kyou has been dead since episode 5, and if I'm not mistaken, Mamoru sees Sena's dead body in episode 12. (Even if he doesn't, I find it hard to believe that Aya would not have mentioned to Mamoru that Sena died in his arms.)
3. Sena and Kyou still counted as part of the third generation, since Aya, Ken, and Yohji didn't retire before they came along. The mysterious fourth generation is a group of young men who are recruited and die on a mission during the gap between episodes 12 and 13. The only thing that really troubles me about this idea is the way that Knight reacts to Mamoru's flippancy about the fourth generation's death. I doubt that Knight's really concerned about anyone but Aya, and I think if it were someone else's death in question, he either would not have reacted as strongly or would have guided the conversation in a slightly different direction (asking different questions, etc.).