From Zero 4, Zero keeping his promise to X up until the end.
Andrew's backstory. He's a reploid in the Resistance who looks like an old man, despite that fact that reploids don't physically age like humans do. It turns out he did originally look younger, but then he fell in love with and married a human girl. As she grew older, he would periodically alter his appearance to look the same age she did. When she finally passed away, he kept his elderly appearance as a way to always remember her.