the letter 'o'.
My family is everywhere,
Giant and small, as fitting,
I rarely live just by myself,
With my kin I'll be keeping.
Letters are everywhere, capital and lowercase, and generally found in groups.
Between the twins when I need help,
Between the twins when laughing,
Beside the knight when I agree,
OK (the knight = Kay, one of the Knights of the Round Table (via @GarethMcCaughan))
Beside the knight when falling.
KO (boxing, knock-out)
Here I'll be seen with my bare head,
English o's (usually) have no diacritical marks.
Away I'll wear my helmet,
Non-english o's often do. (https://en.wiktionary.org/wiki/Appendix:Variations_of_%22o%22#Diacritics)
My presence is quite rarely shunned,
it's a common letter.
But still, we haven't met yet.
There are no o's in the riddle or title!