As far as I know there are a lot of cases where a new born baby is left behind and even as they are feed every hour or so they die because nobody actually loves them and nobody shows them affection. So what if fushimi actually had a nanny (as a baby) that cared about him but then niki didn't like that or something. And then maybe the nanny and saru meet each other post ROK.
Aw, I like this, baby Saru deserves at least someone who gives him love. So maybe after Fushimi’s born Kisa makes it pretty clear that she has no interest in mothering him, like she takes the absolute minimum maternity leave she can, as soon as it’s clear that her health is good enough to go back to work and the child doesn’t explicitly need her care she’s out. Of course she’s not going to let the kid just die though (that would be a scandal her career might not recover from, after all) and she doesn’t want to leave him with Niki because Niki would probably kill a pet rock. She decides to hire a nanny to watch the child until he’s old enough that she can be reasonably certain Niki won’t kill him on accident. Money is no object of course so she has someone at her company research the best local nannies, in the end she hires this one who isn’t too expensive but is widely considered to be one of the best. The nanny is very thankful to have been hired by such a rich family, like Kisa’s paying triple what the nanny’s normal jobs would and the nanny pretty much has run of the house while no one’s home. The nanny does feel a bit concerned though when she starts because she expected, with Kisa paying so much and apparently having been so keen on getting only the best, that Fushimi would be an extremely loved and doted on baby and instead Kisa pretty much shoves the kid at her like please take this thing off my hands thanks.
Nanny is of course immediately taken with Fushimi, who is a cute grumpy baby with big blue eyes and the cutest face and she just wants to hug him and spoil him forever. Baby Fushimi is fussy but he’s also really sweet and when he laughs and smiles the nanny’s heart just melts, she loves him so much. Maybe she gets to stay with him for a while before everything goes bad, like she’s there when he takes his first steps and says his first word (‘monkey,’ one day after Niki showed up unexpectedly to see if his kid was still alive) and she helps potty train him and basically slowly falls in love with this cute sweet kid. She sees him a little less once he’s not a baby anymore and maybe that’s what starts to make her a little uncomfortable, like Fushimi is spending more time that he should theoretically be with his parents but the nanny never sees Kisa there when she leaves the house and little Saruhiko seems to shrink back whenever Niki appears. She’s really starting to worry because she thinks that Fushimi is a sweet quiet child and seeing these little stirrings of fear and misery in his eyes every time she leaves makes her worried.
Then maybe one day Niki shows up while the nanny’s there and he sees for himself how attached his little monkey is to the nanny. Naturally Niki can’t have that, Saruhiko’s much cuter when he cries after all and he wants to make sure his son realizes early that anything he gets attached to can be destroyed by Niki. Maybe Niki plays some really awful prank on the nanny too, like he ‘accidentally’ causes her to fall off the balcony at the top of stairs and she ends up in the hospital and it’s considered lucky she lived at all. The nanny knows that Niki was the perpetrator but of course the whole thing has been covered up before she even gets out of the hospital and it’s considered an accident due to her own negligence, maybe Kisa even shows up (having no idea that Niki was the cause) and tells the nanny that she’s lucky Kisa isn’t pressing charges for endangering Saruhiko. The nanny is all devastated because she knows she’s probably leaving little Fushimi to something terrible but she’s too traumatized and terrified of Niki to try to go back to that house ever again.
So then years later post-ROK the nanny is still working as a nanny and she’s never really forgotten that sweet sad little child she had to leave behind, not being able to see Fushimi again has been one of her biggest regrets ever since. She’s working for some other rich family taking care of their kids when she and the children get caught up in like a hostage situation involving some of the Strains still running around. They get saved by Scepter 4 and in the aftermath she’s calming down the frightened kids when she hears someone yell for a 'Saruhiko.’ The name immediately brings back memories and she looks over to where this red haired kid who showed up midway through the rescue mission to help is running up to talk to this young man with striking blue eyes and glasses. The nanny tells herself that there’s no way, it’s not like that’s an entirely uncommon name, and that’s when one of the random Scepter 4 guys also walks up to the blue-eyed man and calls him 'Fushimi-san.’ The nanny immediately feels this rush of relief and as Fushimi walks by to check out some aspect of the crime scene she calls to him. Fushimi looks up at her blankly, asking if she knows him, and the nanny smiles sadly as she says she’s supposes he wouldn’t recognize her after all this time. Maybe she like starts to sing some small made-up lullaby that she used to sing to baby Saruhiko and Fushimi’s breath just catches because he’d repressed a lot of those good memories, say he saw what Niki did to the nanny and the last time he saw her she was lying injured at the bottom of the stairs and poor tiny Fushimi was traumatized thinking that the nanny had been killed because of him. But now all the memories come rushing back and he just falls to his knees and quietly says like this cute pet name that he used to refer to the nanny as and she smiles and gives him this warm hug, saying that she’s so glad that he’s grown up so well.