The answer changes a bit depending on the s/i, so I'm gonna start with more general and like personal and then go into my s/is.
Dazai and I both, let's say wrestle with certain feelings about life and the world around us. When it feels like you're adrift and very little matters, it helps a great deal to have someone to ground you, and that's Dazai and I for each other. When the world is cruel, we can reach out our hands and feel the warmth, and through that feel safe and real. Neither of us are used to letting ourselves be fully vulnerable to the people around us, we're always masking something, maybe not for entirely the same reasons, but we are. Our relationship is when we both let those walls fall bit by bit, letting go of fear and trepidation. He never even thought he could feel real love before, both of us had thought we weren't worthy of it.
And Dazai is a genius, he's amazing and confident in his abilities, he can see straight through pretty much anyone, so I think why would he even want me, I'm so boring and predictable, I'm not a fan of taking risks and putting myself out there. But Dazai sees all my good qualities, he appreciates my strong empathy and my compassion, he sees how even though I struggle with my own worth I put so much worth into the lives around me, and I have a fun way of thinking about a lot of things, I'm creative bright even when I think I'm not. I become like a fresh, sunny spring day for his soul. And Dazai, his hands are so stained and his mind so jaded, even as he works to redeem himself and be on the side that saves people, he thinks there's too much darkness to ever truly be washed away. But I know that even though he's done a lot of bad, what matters most is what he's doing now; he's trying so hard to be good, to move away from the darkness that only acted as a negative feedback loop for him, that was never good for a boy with a mind like his. He is actively trying to be a good man, and I remind him of that. Neither of us are ever going to be perfect, but that's fine when we're together through our flaws. And through it all, we help each other see the beautiful things to live for.
Okay, now let's go into some specifics for the s/is.
ADA! Gillian has been through some pretty deep trauma with the loss of her little sister when they were kids, and at the time she thought she'd never ever recover from that and stay in the emotionless darkness forever, but with the help of Fukuzawa and Ranpo and the other agency members, she was able to heal. Despite the guilt and trauma that still sticks to her, how easy it would be to write the world off as simply cruel and uncaring, that's not her style. She loves the world, she loves the people in it, she knows that there's darkness but that only means that the rest of them should try their hardest to spread as much love and compassion as they can to balance that out. To Dazai, her unwavering light is strange but so calming. She's so strong in her determination to protect her family and everyone and everything that needs saving, it really touches something in him. She teaches him that it's okay, that they deserve to laugh and love and live, and she helps show him how to actively view the world for it's good parts. Even if someday it's hard, some days she's sad and can't forget the past, some days she tries very hard to push away the anger that festers in her at the unfairness that abounds, she still tries and now they can stand by each other's sides and try together. And he also knows what it's like to suffer and lose the one person who's most important, and he helps her confront the guilt that still clings to her, in fact that's something mutual. And she also, even though she accepts and appreciates her ability for how it lets her help people, it's also an ability that takes away a person's free will and can cause a lot of destruction, and she is afraid of the inherent evilness of it, and though Dazai respects how she's made the concious choice to only use it to help people, he sees her fear and helps her accept it.
Mafia! Gillian and Dazai probably have the most complicated relationship of all. Neither really wanted friends or saw the use of them, but they became each other's first real friend after he joins the mafia. They connect and resonate in a much stronger and more natural way than either were really prepared for; and then they were part of the quartet with Ango and Oda too, and she loved them all. She could be quoted as saying the three of them were probably the only things keeping her sane in the Port Mafia. And then she went away on a mission for a few weeks, no contact with her friends, and suddenly that little slice of joy she had was shattered, Oda was killed, Ango had been a double agent the whole time, and Dazai had abandoned her without so much as a good bye, much less and explanation. It sent her to a dark place for a while. She wanted to hate Dazai, and she certainly felt bitter, but she couldn't bring herself to hate him; how could she, really. She disliked being in the Port Mafia, but not only does she feel she'd have no where else to go, that if she left she'd be leaving her father, Ougai, aka the only person who's ever seen to genuinely want her around and stay that way, but her ability is literally to control darkness and too much light literally causes her pain and discomfort, it's clear to her that she was born to forever stay in the world of darkness and never be able to stand in the light. When she and Dazai eventually meet again four years after he left the mafia, there's a lot of complicated feelings too work through. She's bitter and angry and can't understand why he'd leave her like that if their friendship really meant anything; Dazai thought it was the right move at the time, he justified it to himself by reasoning that he knew she felt chained to the mafia and he had to leave quickly and cleanly in order to successfully rid himself of his dark past, he couldn't risk waiting for her to come back from her mission and having to convince her. But, really, he was afraid. After all, he's convinced that everything he desires will slip through his fingers the moment he obtains it. If he tried to hold on to the happiness she brought him and selfishly took her with him, he'd only bring her ruin some other way, and he wasn't deserving of her. He genuinely does regret it though, and it's not easy for him to admit that he was wrong but he knows that this is one instance where he was so terribly wrong. They have to work through these feelings in order to get anywhere, and she also has to realize that she does have the capability to step into the light, which she does partially with Dazai's help. There's a lot of fighting through the bullshit to finally be together.
Jekyll! Gillian takes the stuff mentioned earlier about always masking some part of ourselves to the extreme. Her ability, Dr. Jekyll and Mr. Hyde, literally manifests her jaded view of the world into a physical creature of chaos, aka Hyde. And she rejects Hyde hardcore, that's why she's so unstable, destructive, and difficult to control, as well as hates her in return. She puts on the sugariest of sweet faces to try and mask this darkness, but Dazai is able to see it. He recognizes her mask easily, because he's basically doing the exact same thing. Eventually, after a lot of plot haha, they're able to help each other let go of their facades a bit and better accept themselves for who they are. They find this kinship in each other that honestly makes it easier for them to let go of their guards at least a little bit. They both hold a lot of jaded darkness with themselves, and they've both done some pretty terrible things and dirtied their hands, him in the mafia and her in the Order of the Clocktower, and they were both able to break away from that to try and become better people, and that's really nice for them to be able to relate to each other.
Circus! Gillian is, true to the name of her troupe the Circus of the Disillusioned, disillusioned about much of the world. It's dirty and cruel and not on your side. But, the circus always promoted family, the whole reason Voltaire formed the troupe was to attempt to not lay there and accept their wretched fate, that they as humans should try and create at least small pockets of a world more right and colorful. And this ideal stays with her. So yeah, they're both not huge fans of the world, but she has a more innate desire to change that, and she believes it's the duty of humans to fight through and not back away from the world through means like suicide (does that make sense? Trying to word it properly). So she actually is pretty, hm, disgusted is too strong of a word to use, she clashes a lot with Dazai's suicidal jokes. And she's too tsundere and jaded herself to outright be all flowery ~I will help you~, but that sort of discussion is a theme between them early on. Their abilities are foils for each other as well, Dazai is an ability nullifier, she's an ability amplifier, and that sort of reflects their views too.
Guild! Gillian at first seems to have the most innocent view of the world, after all she's rich and spoiled by her father, Francis. And she acts rather carefree too, like someone who's always been secure and never known difficulty. But she has known pain, and there's more than a naive rich girl beneath the surface. She's cunning and knows how to read people, she's been trained in the art of business since she was a child and had it drilled into her that you must never roll over for the world. She's also been taught that she's the daughter of the great Fitzgerald, which means she's meant for greatness too, and she hides it from her father but that's left her with a desperation to prove herself and live up to a great big shadow. But she's genuinely kind too, she loves the world for it's flaws and wants to support the people in it. So yeah, they're ways of thinking clash a bit, but at the same time they work perfectly in other aspects. At first, it's more like he's interested in her for the sort of contradictions she poses, but he starts to genuinely respect her and admire how she chooses to see kindness and work for it, how she takes things in to her own hands to make the world she sees in her mind real. And she respects him for his intellect and eventually for his resolve once she learns of his past. And respect is pretty much the bud that will bloom into love.
There's a lot of fighting to find the light in the dark and acceptance of ourselves.
I hope this was all coherent and not to rambly ha.
