Alter Ego / Identity Swap / Mistaken Identity

FL must live as a boy until she is 26, but on occasions when she dresses as a girl she pretends to be her "sister"

FL takes on her friend's identity

ML has a work identity (ala superman) and an under cover alter ego (ala Clark Kent) and is still trying to figure out who he really is (ala Kal-El)

The ML enters an alternate timeline where an alternate past version of himself exists. He uses a mask to take on a false identity so he can help his past self and the FL without exposing his identity. He sometimes removes the mask and takes the place of his past self, particularly when necessary to protect the FL. 

Sister uses her brother's identity

FL's ex makes a robot that looks exactly like her. A series of circumstances leads her to pretend to be the robot. 

The FL has several identities that she keeps separate and secret from different groups: 

1. her true identity as a beautiful heiress, her natural volume / personality is somewhere between #2 and #3 but varies based on the situation (Chen Qingqing)
2. her under-cover identity as an "ugly" and poor student (Chen Qingqing + wig / makeup / glasses / bold voice + attitude / colourful, comfortable, and childish clothing)
3. her identity as a beautiful and polished girl who appears from time to time (Qing Chen - always in beautiful clothes + makeup + her natural hair + soft spoken voice), and 
4. her identity as an online comic artist / an idol's #1 fan (Hebiancao - online alias). 

The ML is a robot copy of his creator's human son. He switches places with the son.

FL dresses as a boy and uses her brother's name, but occasionally has to revert back to her own identity.

Twins are mistaken for each other in childhood and after an accident

FL asks her friend to take her place / pretend to be her in certain situations

When the FL goes back in time she takes on the identity of a local village woman's son who was killed in battle. Later on she pretends to be a princess to save the real princess from people who are trying to kidnap her. The ML later pretends to be the princess' brother. 

FL changes her name and lives as a commoner with her adoptive father; due to a series of events + the ML's amnesia, the ML ends up taking on a new name and identity with a fake back story.

Twin takes her sister's place

Twin sisters switch places

The FL and a girl who was in the same class in school and who now works in the same office are frequently mistaken for each other

Twin sister fills in for her brother

ML has multiple personalities / alter egos

The ML is mistaken for the FL's estranged and deceased brother who had the same name, and due to certain circumstances he runs with it.

ML is not aware of his true identity + takes on the identity of the FL's brother

Younger brother takes his older brother's place