Elena fits the Light Feminine And Dark Feminine trope, especially with Katherine. Elena and Katherine are both doppelgängers who share an uncanny physical appearance. Elena is the Light Feminine (good, moral) and Katherine is Dark Feminine (bad, immoral). Both share a common love and interest in Stefan and are often in competition with each other for his affections.
Elena is definitely a Nice Girl. She is said to be the complete opposite of her doppelgänger, Katherine.
Elena is sometimes called or referred to as a Mary Sue. She is often compared to Bella Swan from Twilight, who is also frequently called a Mary Sue.
Her love triangle is of the "Betty vs Veronica" type (with Elena as "Archie", Stefan as "Betty" and Damon as "Veronica"). In this scenario, Elena (Archie) is torn between her soulmate Stefan (Betty) and her unpredictable, wild friend Damon (Veronica).
Elena is also involved in a "Sibling Triangle" (with Stefan and Damon being brothers in love with her).
From seasons one to three, Elena was in a Beast And Beauty type of relationship with Stefan, with Elena as the Beauty (a young, beautiful human girl] and Stefan as the Beast [a century old immortal vampire). Elena is often the key for enhancing and bringing out Stefan's humanity.
Elena and Stefan could be classified as High School Sweethearts since they developed a romantic relationship in a high school setting, despite Stefan being over a century old.