in andrewās life, thereās a strict system of access. neil made his way into this system not because andrew loves him, but because he went through hell, didnāt betray, accepted the rules, and never questioned them. if andrew says thereās nothing between them, neil silently accepts it and understands that labeling their relationship is pointless. katelyn is an additional variable in his system, representing something from the world of normal people. he sees her as someone who wonāt be able to cope with their reality. the desire to get rid of her is a normal reaction for him; thatās exactly how his primitive defense mechanism works.
besides, katelyn is aaronās attempt to pretend to be normal, but he isnāt. aaron is just as broken as andrew. his girlfriend is a symbol of that ānormal lifeā that andrew hates, because itās all a lie. andrew knows that in a critical situation, katelyn will either run away or break down, whereas neil wonāt. thatās exactly why he trusts neil unconditionally but disregards katelyn. until she proves sheās trustworthy, itāll stay that way.
the truth is, katelyn doesnāt care about anyoneās approval. her goal is to be with aaron, not to win the blessing of his strange, silent twin brother, who looks at her as if sheās invaded enemy territory and is walking through a minefield. she doesnāt need to prove anything to anyone. itās enough that aaron chose her.
andrew doesnāt know what sheās really like? she doesnāt care. andrew interpret her unwillingness to prove anything to him as a sign of weakness or indifference? katelyn considers his demands an absurd intrusion into her private life.
they mean nothing to each other, and if he wants to continue treating her like an enemy, so be it. katelyn knows thatās not true, just as she knows that itās enough for her that aaron will always take her side, just as she will take his.