I doubt there is the "strongest character in fiction", since every verse uses his own cosmology and definition of power.
Does "no reality warping" means other sort of hax too?
However, if we going with the strength or the power to destroy entire universes with power there are some people in my mind:
1.) Mars Demonbane destroyed universes with the expansion of his body. His size and his presence alone brought the universe to collapse. In a fight against Liber Legis, one strike destroyed billions of universes.
2.) Gabramelek from the web novel Soul Cartel has the power of 12 universes and can use the entire force against his enemies.
3.) Virgo Shaka and Virgo Asmita created and destroyed milliards of universes (Depended on the translation countless universes) in Saint Seiya Next Dimension
However, they are fodder if it comes against some high level reality warper, or even conceptual manipulators.
Beings like 80% of the Umineko cast could shit on them, the same with Outer Gods from Lovecraft, The Endless from DC/Vertigo, Magic Gods from To Aru Majutsu no Index, Magi Gods from Magi, etc.
But a multiverse contains a shit ton of universes instead :)
If we talking about an average multiverse, then we have infinite universes (space time continuums). That is already far above Zeno his capacity, but not really impressive in fiction.
Tought Robot Superman murderstomps even the stronger visual novels which are Omniverse Level (like Umineko etc)
This boy have Reality Warping, Conceptual Manipulatoin,Inmortality,Beyond-Dimensional Existence and even resistence to Plot Manipulation.
CAS has also Plot-Manipulation, which allowed him to beat his enemy because he can. He is a walking NLF machine and there are not much characters who can beat him.