After Experiment Number Eighteen, Zeriel and Zarethra as are now known as, start their new adventure together in the surface.
From the very beginning, both are together deeply. But Zeriel is merelly aware of few things, unable to remember anything and just be aware.
Since the beginning, Zarethra owns the soul, the determination, is the main determination production for the soul, owns the RESET, can take control over Zeriel when Zeriel relaxes and drops control even a little, and can take Zeriel to see a future or back home to the Void or see the perspective of himself in other timeline when he goes to sleep as is when he dies as the soul is taken out of the body.
With the pass of years, Zarethra's plan to have the best future where Zeriel is how she desires and pleases, goes playing. At first, is unknown if she has replayed this already, but is possible she replayed everything hundreds of times, changing timelines and taking Zeriel along, or acting in ways Zeriel does not understand.
In the process, Zeriel gets more and more aware, but also acceptance as knows he can't do anything, and even learns to enjoy being under Zarethra's control.
Zarethra's plan:
1st step - Zeriel learns how Humanity truly is.
2nd step - Zeriel learns to change and builds his new self in a new shape, the shape he has to have.
2 simple steps prolongued further beyond into much more complex.
First, Zeriel learnt, but got infected with Hate, which then would try to overcome him, but with Zarethra's determination, and his own refusal to prevail the pacifist route, reaches to contain but with a cost...
Despite Hate did affect deeply, Zarethra took it as an opportunity, and used the Hate to reinforce the shape into an even better shape.
A shape of her own, and that's why the visual changes over UltSans' hair and pupils, apart of behaviour.
With the pass of years, and by everything that has happened, the only one who knew the future, was still Zarethra, but now Zeriel was more as she wanted him to be, and was getting him ready for when was really required to enter the action.
Everything, all the suffering and pain he endured, everything he went through, were just trials Zarethra forced him to go through, so he would change and learn by the hardest way, and would be rewarded if he really learns, and so does rewards him if he shows he is learning and changing.
All of this truly plays for the Canon versions, while for the Fanon versions changed this control and played their own way, and Hate's game, and are now in a strange acceptance redemption, while the Canon versions remain playing under Zarethra's control and re-shaped.
Their relationship is strange and seems forced. Zeriel doesn't really love her, as his love is entirely based on his neediness. And Zarethra is toxic and possessive.
Zarethra does anything to make sure no one that is not even the least relevant for even the smallest thing inside her plans, takes out instantly without hesitation. This confirms all the girls Zeriel did had a relationship, were taken out by Zarethra to make sure Zeriel learnt to stop being as how Asriel was originally, and to strength him and his personality. Also confirms is behind what happened to Lily and Cad, taking both out to steel and strength Zeriel even more to the very limit.