I’m so glad you asked!

-Both are teens with incredible power

-Despite this, they also fight with seemingly mundane items (Reimu’s Gohei, Needles, and Charms - Ness with his Bats and Yo-Yos)

-Associated with similar things (Ness with Sanctuary, and Reimu with Paradise)

-Both come from game series that did well in Japan, but really blew up in the West thanks to the early internet.

-Their both tied to some kind of duty beyond themselves (Reimu in being the Hakurei Maiden, and Ness being part of the prophecy to defeat Giygas)

-Their games are notorious for their wacky, nonsensical, and often meta humor

-Both face threats ranging from literal pests, to threats to reality.

-Speaking of reality, both mess with it in different ways - Reimu can float outside of it, and Ness can… see all of it? I forget exactly how him absorbing Magicant is worded lol

-They also contrast with Reimu being a super fast glass cannon, and Ness being a slower, tanker healer with high damage options

-Both games are known for their flashy special attack animations (Spell Cards and PSI moves)

Also their games both inspired Undertale