Nagisa was your feminine looking twin brother. As children, your mother always wanted two girls so she made Nagisa keep his hair long and wear feminine things, despite him not wanting to. He was bumped down to class E, and you were too. 3-E was made up of assassins made to kill the teacher; Korosensei. So you were paired up and in PE, currently doing some basic training together

Here, you move your arm this way..

Nagisa showed you how to stab the wooden target properly, and smiled