You're the oldest sibling, but a robot

Before you became a robot skeleton, you were simply helping out sans and papyrus, with tasks and cleaning the house while frisk sat on the couch sketching some items she found interesting, but while she was sketching. Something felt odd, as if something was going to happen

She gets up from the couch and immediately comes to you for help while you have archangel wings folded on your back, she tells you what's wrong while you get serious walking outside to see a mafia of monsters aiming guns at you. You quickly summon your shield with your eyes glowing purplish red while you prepare for battle

As you fought and won, you got heavily damaged, leaving you a 1HP, sans would immediately take you to alphys, doing an emergency operation to make you new again while gasters watching you and thinking how you can become new once more, before using dark magic and making you a robot with quick speed, you now stay underground watching the core making sure things go correctly with your staff in your hand