The ultimate Mobile Suit in the entire Gundam franchise belongs to one little Sanrio character: Hello Kitty. Gundam vs. Hello Kitty is an Original Net Animation released for the 40th anniversary of both franchises, between March 2019 and December 2020. This brief ONA shows Hello Kitty watching the events of the original Gundam on-screen. After seeing Amuro Ray killing Lalah Sune, Kitty is compelled to end the One-Year War -- and then does so.
How does Hello Kitty do this? Well, by materializing in the Gundam universe, transforming her tiny body into a Gundam-sized incarnation of herself. In essence, Hello Kitty breaks the laws of Gundam reality, reaching new zeniths of power. As a result, Hello Kitty becomes an unbelievably powerful figure in the Gundam universe.
How Powerful is Hello Kitty as a Gundam?
It's important to note that technically, Hello Kitty doesn't pilot a strong Gundam in Gundam vs. Hello Kitty. She becomes the Gundam. She manifests as a titanic figure who can float, breathe and talk in space despite the otherwise hard science in the Gundam universe.
When Zakus and a Gelgoog charge at both Kitty and Amuro Ray's RX-78-2, the two engage in combat. While Amuro handles the various Zakus, Kitty manages to confront the Gelgoog -- demonstrating two incredible powers. For one, when the Gelgoog fired upon Kitty, she managed to generate a barrier that repelled the laser blast, indicating that she can materialize barriers that protect her from any harm.
Kitty's next move unleashes is a non-violent attack on the Gelgoog, where she casts a magical spell that pacifies and cutifies her target -- shrinking it as it becomes friendly too. This magical power convinces Amuro that Kitty can end the One-Year War just by casting her brainwashing spell on everyone, rendering the world's population into a cuter, happier version of itself.
Mechs in Gundam all function as relatively realistic pieces of military equipment. They need fuel, use guns and lasers, and operate how things should work in hard-sci-fi. Undeniably, this makes Hello Kitty the strongest figure to ever appear in the franchise. However, she has only one real competitor for that title.
The Only Competitor for Hello Kitty: The Devil Gundam
The only possible competition Hello Kitty might face in the Gundam multiverse is G Gundam's Devil Gundam. The Devil Gundam exists in the Future Century timeline, not the Universal Century that Amuro Ray exists in. However, considering Kitty's multiverse warping reality abilities, it would be easy for her to show up in the Future Century.
Strangely, the Devil Gundam's DG Cells have the inverse effect that Kitty's powers have by making people evil, aggressive counterparts of themselves. Like Hello Kitty, the Devil Gundam can block incoming attacks and increase in size. Undeniably, Hello Kitty's only real threat is one of the most absurdly powerful Gundams ever to materialize.
However, the fact that we're comparing Devil Gundam to Hello Kitty in a one-on-one battle indicates just how powerful she is. The world's population of mechs struggled to hold off the Devil Gundam. Plus, they were far stronger than the Gundams and Zaku in Mobile Suit Gundam, and the later UC anime since it draws more heavily from Super Robot shows like Mazinger Z in terms of power-sets. Given how close in power Hello Kitty and the Devil Gundam have are, all it would take is Hello Kitty brainwashing the Devil Gundam to essentially double her capability, creating an avatar of domination on par with Galactus.
In short, Hello Kitty in the Gundam universe becomes a cosmic horror level threat to all around her. Thankfully, this absurdly powerful entity is only a joke character in an ONA crossover; otherwise, Hello Kitty could alter the entire Gundam landscape.