-Two trickster, backstabbing, ever-so-smiling and reality-warping antagonists from RPG games who at first don't seem like they would play big parts of the story, but where later revealed to be the true main antagonist of their story all along.

-Both watch the protagonists throughout the game behind the scenes and occasionally pop up to toy with the protagonist of said game.

-Both seek the power of a heart-shaped McGuffin(s) (The Chaos Heart and the Human SOULs) to cause the destruction of reality, as well as having some funny business with a set of seven rainbow-themed hearts (The aforementioned Human SOULs and the Pure Hearts), which ultimately lead to their downfall.

-With the power of said hearts (added connections with both infusing themselves with other beings, being Luigi and the SOULs), they gained a much bigger, more powerful and more terrifying super form which threatens all of existence (Super Dimentio and Photoshop/Omega Flowey), with these forms in a way being an amalgamation (Super Dimentio actually being a direct fusion of Luigi and Dimentio, and Photoshop Flowey being an amalgam of various images and sprites photoshopped together), as well as being practically nigh-unkillable until the coloured hearts came in (Super Dimentio literally being invincible and Omega Flowey having a very high defense).

-Both also kinda caused the destruction of a reality (realities in Dimentio's case) after obtaining said hearts, but after their defeat the damage was reversed and the worlds were saved thanks to our heroes.

-Both used the younger, slimer and taller brother of their game's most popular character as a pawn in their game, and eventually due to them they managed to achieve their goals of greater power and destruction (Paper Luigi and Papyrus, with Dimentio brainwashing (but not really) Luigi to make him Mr. L and use him as a minion to do his beeding, and used him to become Super Dimentio, and Flowey getting on Papyrus' good side and trick him into bringing the UT cast together, which unintentionally or not lead him to becoming Asriel)

-Both decided to go out and kill (or, at least attempt in Dimentio's case) a cloaked, broken man who were once thought to be the true antagonist of their story whom they are pretty closely associated with (Asgore and Count Bleck, with Asgore being Flowey's father, and Dimentio serving as a minion to Bleck), to which this way they managed to obtain their aforementioned heart-shaped MacGuffins.

-Both them and their games have a pretty big association with prophecies and have in some way fulfilled said prophecies, even if it was short lived (Dimentio being associated with the Dark Prognosticus and pursuing the end of all worlds, while Flowey/Asriel eventually became the Angel in Undertale's ancient prophecies, possibly not counting Genocide Frisk).

-Both inexpicably, except not really, have the same unique-ish ability of the protagonist of their game (Mario's Flip technique and Frisk's SAVE/LOAD powers) which at first they didn't have (Mario was actually granted this power by another person and it seemed that prior the falling into the Underground, Frisk didn't have this ability).

This isn't so much of a connection as it is just a neat thing, but funnily enough Dimentio uses a plant, the Floro Sprouts, to mind control people, and Flowey is... Well, a plant.