Tensa said: It's her fathers ... Keine can't die of old age...
Youkai and half-youkai like Keine can be killed. If Mokou is killed, she will always resurrect. As well as that, youkai aren't ageless, simply very long-lived by human standards. A million years will be less than a blink of an eye to Mokou. One way or another, unless their relationship ends/sours, Mokou WILL end up mourning her.
Tensa said: It's her fathers ... Keine can't die of old age...
Even youkai die of old age (and Keine isn't even a youkai, she's just a were-hakutaku). Mokou is immortal and ageless, Keine WILL eventually die and leave Mokou by herself.