WMG / Saber Marionette J

Otaru is a illegimate clone.
We do know the "fake" Faust clone from JtoX. What if Otaru was made for a similar purpose, to be a Puppet King to replace Ieyasu in secret, but he somehow got out when he still was young!

The Saber Marionettes of Japoness are real women
Otaru's dreams usually involve the likes of his Saber Marionettes, Lime in particular appearing long before Otaru knows who she is or what she looks like. Later on in the J series, he sees his two other marionettes alongside a different-looking Lime in the same school setting. Considering his uncanny resemblance to the original Tokugawa Ieyasu and his lack of actually going to school past the primary years, perhaps Otaru has a stronger connection to Ieyasu than Ieyasu's direct clone. Perhaps the original Ieyasu influenced the design of the three marionettes based on three different schoolgirls he encountered while studying, the three girls who would later share traits with Lorelei, his love interest aboard the Mesopotamia.

Otaru is the true essence of Ieyasu
Despite being born in a planet without women, Otaru sometimes remembers having a mother. Some theories say it's because he's still one of Ieyasu's clones and he being the only man having marionettes who behave like women made those lost memories come to surface under certain events.

Another theory implies this is the reason he's the only man in all Terra II who considered marionettes as more than just machines as he "remembers" some real women and unconsciously he can't help but see the marionettes as real.

Marionettes are full replacements for the human female
It's established fairly early in Saber Marionette J that the marionettes Otaru comes across have the ability to mimic basic bodily fluids (tears, blood). How? They eat food, the basic nutrition that a human body requires to produce its bodily fluids. The marionettes need to have some sort of "digestive tract" in order to properly convert their food into the human-like bodily fluids and energy. This ability comes into play in J to X, where Cherry struggles with her inability to become a real mother to her "adoptive" son, whom she finds lost out in the open. In a scene very reminiscent of a mother breastfeeding her child to make him/her stop crying of hunger, Cherry manages to stop her baby's crying by using the motherly method.

Later on, the issue comes up again when Lime and the others start having existential crises in Xi'an. Cherry works at a local religious orphanage, but then she's told by one of the kids that she can't properly take care of the babies' "maternal needs", roughly meaning that she can't feed them herself and lacks actual breast milk. Because the marionettes are built so similar to a normal human body (sans robotic limbs), the last claim becomes dubious in the fact that a grade-school kid would know what breast milk is and how it works, as well as striking evidence to confirm the marionette's "unnecessary" bodily fluids, contradicting itself in one fell swoop.

Or Cherry's assets are merely pacifiers glued to her chest.