Trang Pak is one of the Cool Asians at North Shore High School, and is believed to be the clique's 'leader'. While her character doesn't actually play any significant role in the movie's main plot, she has shown to be really bossy and arrogant.
Even though the film makes no mention of her having any siblings, Tina Fey confirmed in the commentaries that Trang Pak is actually Tim's little sister.
Trang Pak makes her first appearance in the cafeteria, along with the rest of her Cool Asians clique. As Cady was walking pass her table, Trang gave her a cold, judgemental look and placed her bag on the empty seat so Cady couldn't seat there.
After Regina published the pages of her Burn Book (which said that both Trang Pak and Sun Jin Dinh were romantically involved with Coach Carr) the two got into a vicious cat fight, before Principal Duvall stopped them. Afterwards, Coach Carr reportedly flees school property, thereby confirming his guilt and at least one of the allegations in the Burn Book.