Papyrus is definitely not oblivious to how much Gaster has changed, either.
One day, you're taking a break, leaning back in a comfortable office chair you scrounged from the dump, in the workshop. Papyrus is defying gravity, as he is literally climbing the walls as he runs up and down the walls and ceiling. Sometimes finding ways for him to burn off his hyperactive energy can be challenging, you muse thoughtfully, as you watch your younger brother and his amazing antics.
"Sans," he says, panting as he runs up and down. "I - ", puff, "can't help but notice," pant, "Dad is a little..." pant, pant, pant, "...sad?" He stops, upside down on the ceiling, as he says this, looking at you inquiringly. But stopping makes him lose his concentration on his magic, and he falls with a yelp.
You catch him in mid-air and set him down gently with blue gravity magic, with a skill born of long practice. He doesn't even look surprised, having taken it for granted that you would, and once upright and on solid ground again, he continues asking you the question with his eye lights and inquisitive, determined expression.
You sigh. "yeah, bro. he's...not very happy lately."
"Do you...do you not think it is encumbent upon us to ease his distress?" Papyrus asks, trying out some big words he learned recently. He loves playing around with language like that. You think he might've inherited it from Gaster.
"i'm not sure what we can do, to be honest," you tell him. "i've tried asking him. i've tried asking alphys. heck, i've even gone all the way to asgore's house and asked him. i think it's something to do with his research, but no one will tell me anything."
"I don't want him to be sad," Papyrus says. "I want to help! Do you think if I did something for him, that would help him feel better?"
"yeah...maybe, bro."
"Then that's what I'll do! I know! I will make something spectacular and stupendous and amazing for him! Then there's no way he can be sad!"
You don't have the heart to tell him that's probably not going to be enough. Maybe that's partly because you have the faint hope that Papyrus will somehow be able to get through to him.
You try your best in the following days to keep Gaster's increasingly unpredictable moods from affecting Papyrus, but every time Gaster snubs him, Papyrus's voice grows a little quieter, and his naturally ebullient personality dims a little more. You feel you have to do something before Papyrus loses who he is completely.
One evening, Gaster is working at his desk in the living room when Papyrus approaches him with a drawing.
"Um, Dad...? I made this for you..."
You drift closer out of curiosity. It looks like Papyrus drew a crayon drawing of himself holding hands with Gaster. It's kind of sweet, and he obviously put a lot of effort into it with all the colors and lines.
Gaster looks up from his paperwork, takes a quick glance at his drawing, and then tosses it back to him. It falls haphazardly to the floor.
"I don't have time for games, or presents. You know that."
Then he simply ignores Papyrus and goes back to his work, his pencil making scritch-scratch marks on the papers in front of him on some kind of theory of his, the pencil audible only because of the deafening silence in the room as the two of you stare at him in shock.
Papyrus visibly shrinks in on himself and his face falls. You hurry to compensate, not able to bear the look on his face. You grab his hand and lead him away. Gaster seems oblivious.
"come on, bro, i'll look at it with ya," you say to him, whispering so that hopefully Gaster won't hear you and react further. "maybe we can draw some more, ok?"
Papyrus glances back at Gaster before letting you lead him to his room. You shut the door behind you, and he breaks down into silent tears, both of you having learned months ago that crying aloud isn't wise when Gaster is anywhere in the house. You spend hours trying to calm him down, before you finally succeed in cheering him up by getting him to draw some pictures for you, and praising him for it.
Later that night, you confront Gaster about what he did. Lately, you've stopped thinking of him as "dad." Somehow, the word just doesn't seem to fit him anymore.
He's leaning back in his chair in the living room, eye sockets half closed in exhaustion, as Papyrus is sleeping in his room where you left him.
You have the feeling he knows you're there, as you think you see his eye lights focus for a moment on you before darting away, but he's choosing to ignore you. He's making this as hard as possible for you by not starting the conversation himself. You just stand there and stare at him for a long moment before you begin speaking.
"you know, i get that you don't think being a parent is that important anymore, compared to saving all of monsterkind...but if that's really how you feel, why don't you just leave?"
He narrows his eye sockets at you and leans forward.
You feel afraid of him, and his physical stature, and magical strength, but after watching months of abuse you just can't take it anymore. If it was just you, fine. You could deal with that. But you won't let it happen to Papyrus.
You don't back down.
"i mean, you practically live at the lab anyway...i have to make dinner for me and papyrus most nights...you hardly ever ask us how our day was or anything...i bet you wouldn't even notice if we just disappeared one day. most days you just come home late at night, slam the door to your room, and go to sleep like you're dead to the world. then the next morning you leave before we wake up. so why do you even live here? why not just pack up and go live at the lab, since that's where you spend all your time anyway?"
Your voice grows stronger as you draw the line in the sand. You're approaching the mid-teenager stage of skeleton monster development. You're not a child anymore, and he can't intimidate you into submission the way he used to be able to.
"Is. That. So." Gaster says softly, dangerously. There's something dark in him, something you never saw before the day he told you the king was pressuring him to start the soul research.
You steel yourself for a fight, or an intense argument.
"do you honestly think it doesn't hurt him when you treat him like that?"
Gaster's shoulders tense like you'd hit him with those words.
Then something suddenly changes in his whole expression, like a light bulb being switched from red to green.
"They won't stop," he says pleadingly. "They never stop whispering..."
You're taken aback. "what the hell are you talking about...?"
He suddenly grabs you by the shoulders. "Please, Sans. You're my son. You care about me, right? You care what happens to me...? Make it stop!"
His eye lights gaze into yours intensely, but the edges of them are ragged...which you know from studying medical journals for skeletons is a sign of extreme stress, and sometimes mental instability.
His intense focus and the way he's clutching your shoulder bones so strongly that it hurts is seriously freaking you out. You feel like you have to get him off of you now, or else he might actually damage you.
"get off," you tell him with an intense glare. But he doesn't listen.
"I can't take it anymore! Make it stop!" he blabbers incoherently.
Using blue gravity magic, like he himself taught you to do, you grab his soul with a flick of your wrist and throw him off of you, so that he thuds against the wall behind him.
With an unbelievably quick reaction speed, at almost the exact moment you grabbed him, he grabs your soul and sends you crashing against the wall behind you, but with much more force than you sent him with. You feel a distinct crack as you hit the wall.
You didn't push him too hard, so you're pretty sure you didn't hurt him that much. The same can't be said for you. You're pretty sure he cracked at least one of your ribs. But the impact on him seems to switch him back from whatever state of mind he was just in.
He stands up slowly, looking confused. "What was I just talking about?" he murmurs.
"i don't know, something about making them stop whispering? who were you talking about?"
He looks aghast. "I...I shouldn't have said anything."
His eye lights are unfocused and his ribs heave under his shirt as he takes slow, heavy breaths.
"dad, please...can't you just tell me what's going on?"
You almost convince him. You're sure of it. You can see from his expression that he considers it, as he winces from some kind of painful internal dialogue you can see he's having at the same time.
He looks at you closely, in the careful caring manner of the diligent parent that he hasn't been in a long time. You get the sense he's reading your expression and posture and health as closely as you've been reading his. His eye lights gradually become more focused as he looks at you.
"This has been destroying our family, hasn't it?" he asks you. "Me being like this. A total, complete wreck of a monster."
The question makes you assess yourself, and your expression shows a wince.
He notices and gasps in horror. "Did I hurt you? I can't even remember doing that!"
Slowly you hold a hand to your ribs where it hurts the most, making the answer clear.
He shudders and holds his skull in both hands, rubbing his skull so harshly you're sure he's scratching it.
"How bad is it?" he asks, looking up through his hands, as though he's terrified of the answer.
"think you cracked a rib," you mutter lowly. "you were acting weird and wouldn't let me go, so I threw you off me, but then you retaliated and threw me a lot harder than I threw you."
"I see. That answers my question," he says in a tone of intense, bitter self-hatred, lowering his hands. "I've descended to the point where I'm a threat to other monsters. To my own sons. I really am nothing but trash."
"i..." You're not sure what to say, so you take a moment to gather your thoughts before replying.
"look, if this is how you're going to be, i can't let you do this to papyrus. i can take it. he can't. he's too young, and i don't think he can understand why his own father would start being cruel like this to him. you were never like this before. you were a good person before you started some kind of terrible research with the souls. i don't understand how you could change like this either, but i can deal with it if i have to."
He thinks about what you said, and you see his expression become resolved and protective.
"You shouldn't have to," he tells you firmly. "Just because my absence as a stable emotional presence has forced you to act older than you are and cover for me with Papyrus, doesn't make how I've treated you any less wrong than how I've treated him."
"then why-" you start to ask.
"I can't keep letting you be treated like this. It's not fair to you."
Gaster heaves a deep breath and stumbles away from you, towards his room. You follow him and your alarm rises as you see him packing a bag.
"dad, don't just leave!" you plead with him. "i just want some answers! something's destroying you, and i want to know why. please...you don't have to just leave like this..."
He gives you a rueful look that says, "This is what you wanted right? Why are you trying to stop me now?"
You're seriously afraid you're going to be responsible for breaking apart your small family of three by making him feel like he has to leave.
You thought that's what you wanted, if he refused to try to start treating you and your brother with respect and kindness. But now the responsibility is weighing on you heavily, and you're not sure if you're doing the right thing.
"we can get you help! you don't have to leave!" you repeat again.
He shakes his head, refusing to stop packing or tell you what's going on.
"I can't get you involved," he says resolutely. "I won't. Even if I can't protect anyone else, I will protect my own sons from this!"
Then, very strangely, he pauses, turns his skull a little bit away from you, and listens for a moment as if someone is talking who only he can hear.
He suddenly snarls fiercely at them, "Get the hell out!"
It's the only time you've ever seen him bare his true skeleton monster canines at anyone in anger, and you can't even see who he's talking to.
Then Gaster calms down and turns back to you with a sober, serious look on his face. You get the impression he's carefully sizing you up and weighing your age and maturity in his mind.
"You're correct that the current situation is unacceptable," he finally says resignedly. "The truth is, you were right about everything. I'm sorry I tried to make you feel like what you were seeing in me wasn't there. Your instincts and perceptions were absolutely and completely correct. I don't wish for you to learn to ignore them because of me."
You appreciate his well-intentioned words, but you're far more interested in trying to figure out what just happened with him. As far as you can tell, he's switched personalities twice so far, and it was about to be a third time, but he fought it off. How can you possibly deal with a parent, a person in a position of supreme authority over their children, switching personalities from one moment to the next? What would've happened just now if he'd decided to fight back seriously when you used gravity magic on him? You know he could've done a lot more damage if he'd really wanted to, and wasn't just reacting out of instinct.
"who were you talking to?" you ask boldly, mustering your courage and trying to sound confident, even though inside you feel small and scared. Usually you act years older than your age, but his actions and strange behavior have terrified you to the point where right now you feel several years younger. You try not to show it, since at the moment you're the only one who's anywhere close to acting like a consistent, responsible adult in this household, even though you're still a teenager.
"Someone who hates all forms of life," he answers cryptically. "Monsters included."
"what?" you ask incredulously, unable to decipher that. "so what, are you trying to tell me someone is making you be a jerk to us?"
"When is it me, and when is it not me? It's getting harder to tell. If I acted cruelly, and they are cruel, what's the difference? What does it matter which one it is?"
Gaster's voice has started lilting a little too much. It sounds like he's losing touch with reality, and his words are making less and less sense. You notice he's digging the tips of his phalanges into his forearm bones, and there's deep shame on his face.
"it matters," you tell him. "every time you try to do the right thing, it matters."
Sanity gradually returns to his expression after he hears you say that. He notices he's been hurting himself with his phalanges, and sharply pulls away his hand from his forearm with a visible shudder.
He considers for a long time, and then says, thinking aloud to himself, "He's right. My efforts to stave it off have made a difference. I'll keep trying. I won't just give in to it. I won't lose myself in that being's psychopathy, even if losing myself and becoming insensible to reality would make my conscience hurt less. I won't let myself get away with this. I know now the conscience of one monster was too high a price to pay. It had so many negative ripple effects that I couldn't foresee. Are these unforeseen consequences what morals are meant to save us from? Is the best I can do now to make sure the karma falls on me, and no one else?"
He shakes his head, noticing your look of concern and confusion, as you struggle to keep up with what he's saying. He apparently concludes he shouldn't keep talking, to your great sorrow and dismay. You want answers, even if you're scared, and even if the answers hurt your soul. You want the truth more than you want to feel safe or comfortable. You wish he could understand that. You wish he could believe in you enough to try harder to explain what's going on with him.
He finally decides, "I shouldn't be here. I know it's a lot to ask, but Sans, could you take care of things here from now on? I won't bother the two of you anymore. I'll make sure you have enough funds to take care of yourselves. But if I stay here, I might hurt you even worse than I already have."
He finishes packing his bag and hurries to the front door of the house.
"what is going on?!" you yell at him one last time, desperately. Papyrus's door opens, and his little worried face peeks around it. He must've been woken up by all the commotion.
But Gaster doesn't answer. He just rushes from the house, carelessly leaving the front door open to the frosty night. Snowflakes drift down, and your eye lights follow some of them being driven by the wind through the doorway and landing inside, as you stand there, stunned.
"Sans?" You turn and see Papyrus coming down the stairs from his room, rubbing the sleep out of his eye sockets, dressed in his pajamas with space rockets all over them.
"What happened?" He asks nervously, coming over to stand next to you as you both gaze out at the dark night.
"he...he left," you tell him, still in shock. "he just...left."
"He's coming back, though, right?" Papyrus asks, already sensing the answer.
You shake your head, still stunned.
"He's not coming back? Ever?" The plaintive worry in his voice grows.
You shrug, still shaking your head in disbelief.
Papyrus sees how upset you are and realizes what a family earthquake just happened. He tries to fix it.
"There's tracks! See? Maybe we can find him! Ask him to come back!" he says, pointing at the snow.
You nod slowly. "alrite. i want you to stay here and keep the door shut, ok? promise me? i can't risk you getting lost out in this weather."
"I promise," he says eagerly, desperate to be helpful in any way.
You head out into the blizzard, but Gaster's tracks disappear where the snow ends at Waterfall. You do a cursory search around Waterfall. You don't dare search for him for too long, though, because you're so worried about leaving Papyrus alone. He's still a child monster, so you shortcut back to Papyrus within twenty minutes. When he sees you've returned alone, Papyrus breaks down and sobs.
Neither of you get any sleep that night. It's a dreadful night full of tears as the two of you try to comfort each other. You spend a lot of time hugging Papyrus and telling him it'll be ok. You just wish you believed yourself more.
The next day, reality sets in that you're now essentially two orphaned skeleton brothers who have to take care of themselves. You both set to work on learning how to live on your own resources and solve your own problems, without a parent to help you. You rely on each other, and you begin to find ways to cope. It feels like a large chunk of your soul has been damaged, though, and you can tell Papyrus feels the same way. He spends a lot of time just staring out the front windows of the house like he's expecting Gaster to come back.
Later, Gaster requests some things to be delivered to him at the lab with a letter, but he never returns home again.