While she is referring to her best friend/schoolgirl crush/bodyguard Setsuna as "Se-chan" I do not think the person under the translation spell is going to hear that. It seems off.
I know my honorifics well enough to know what it means literally (endearing diminutive feminine title), but I'm having trouble conceiving of a proper American English equivalent. I get hung up on how bad "Sessy" and "Setsy" sound and get roadblocked.
Or maybe it just sounds bad in my head.