I want to know too ;~;
I'm guessing that Katsuya and Suou both exude a seme and uke aura respectively.
As for immortality, I think that they are. It's been hinted/said here and there that carriers ARE immortal. Maybe not immortal but have a longer life span compared to others. Well all the same.
katsuya has the female and suou has the male specter. but it was suou who carried the child specter when he was on the verge of death. so yeah, it's a reverse mpreg. hahaha. it would have been fun it this was longer.
Similar to Seahorses. The male is carrying the child
I'm a bit lost to as why this is tagged as mpreg, when there is no male-pregnancy involved in this story. Do they have a child? Yes, but that child is between the two specters living in their bodies, and completely unrelated to the humans housing them.
As for Souo carrying the child (as in being pregnant with it), it's not accurate. The baby specter was fully developed when the female gave birth to him. Souo only carries him as in being a carrier for him, like for a virus or an illness. Specters are parazytic beings and need a host body to live in, and Souo hosts their "child" after the male specter he previously carried dies due to not being able to whitstand the damage inflicted by Katsuya's childhood friend.
I understand about the "child" part, but what still on my mind are :
1. it may be shonen ai, but I just curious, is katsuya the seme one or suoh?
2. Is suoh and katsuya both gonna be immortal? cuz suoh were supposed to die but was saved cuz of the new specter in his body