Sya insists she is human and gets very upset when questioned about it but she also admits she is "a little more too", so as always i have veeeeeery bizarre theories:
1. Sya constantly refers to Memory as "Mom", the obvious interpretation being, "Syla is a monster and is literally her daughter" this could be the case considering she had a child and monsters can breed with humans, she says she was pure once but not anymore, maybe she only had memory traits but somehow gained some body to shapeshift into a normal looking human and live with her husband, dissapointing Memory who at the time wanted her pure as her monster choosen.
2. Sya is a superhuman and manifested memory so well to the point of being considered an heir, but somehow developed a little of body powers to try to alter herself in a way that allowed her to concieve a child with her husband, if we consider superhuman/human genetic incompatibility shapeshifing could be a solution, but from losing her focus only in her memory trait she dissapointed her monster "mom", wich lead Memory to try to find a 50/50 this time and see how that works.
From what we gather all other apostles only care about pure spawn, so maybe Memory and Body being ok with a 50/50 is a new development.
It's also relevant to remember that according to Sya, Memory DOESN'T know about Mc relation to Syla, that's why she wants him to grow as much as he can in his memory trait before Memory becomes aware of that, wich MAKES NO SENSE TO ME.
Now you guys are into something with Mc's appearance, there is no way he would end up that short when born from people that tall, also memories as we learned are not just in peoples minds but also in their genes, maybe if he has monster heritage blocking that part of him not just erased his memories but also his genetic potencial, until being unlocked by his monster infection, she even says at the mirror scene that Mc's powers only "awoke what was already there".