Used as a term to call someone who hangs on their boyfriend/ girlfriend all the time, thus acting as a "backpack".
This person is characterized by asking for constant piggy-back rides, always hugging, or attached in some way to their boyfriend/ girlfriend. This happens often enough to become awkward to other people near-by.
A Mochila is generally female due to "mochila" being a feminine word in the Spanish language, but is not at all uncommon in males.
*boy and girl come down the hallway for the third time in piggy-back mode*
Student: "Whoa! Here comes a Mochila!"