1) A character from "The Omega Project", a movie series and graphic novel series. She is a Japanese woman that fits the stereotype of the happy anime character and karaoke singer. She ends up marrying the a Black co-main character that often makes fun of her Japanese heritage with the White co-main character (which is the Black guy's partner). Despite this bigotry, she loves him dearly and he eventually learns to love her. Her trademark facial feature is a closed eyed and "D-mouthed" smile.
2) Any such person that acts as cheerful as Mitsuki and/or shows unwarrented happiness in a distressful situation.
1) I'm going to buy teh Mitsuki action figure.
2)Look at that Japanese person over there. She resembles Mitsuki.