Rainbow eyes? TBH, that’s not a super terrible idea, however, some description/justification would be needed. Suppose one could color the eyelashes, or eyebrows, or even the iris itself with concentrically colored rings. Iris/Whites could be done with a contact lens or a potion … in my potter fanfic, I’d simply chalk it up as a party gimmick from Fred & George.
But yeah, the weapon thing would be understandable *IF* they’re military with years of special forces training, and is now a grandpa in a motorized wheelchair. Otherwise, less...credible. Guess that’s the crux of the Mary Sue/Gary Stu thing, it produces a character which is less realistic, less relatable.