She says "I love you" when she calls you some form of a nickname that would be insulting under normal circumstances, but you know her better than that. She's just sarcastic like that, and you've gotten used to her and her witty comebacks. Your acquaintances that don't know her well may look at you strangely, silently wondering if this was "normal," but it was, and you love everything about it. Besides, you've got a handful of good ones saved up to get back at her with.
She says "I love you" when she steals one of your dozen white dress shirts that are too big for her. "They all look the same anyways," she retorts when you tell her to give it back, to which you counter by persistently saying how they were all different in their own ways. She rolls her eyes at your stubbornness and promises to fill your closet with other items that don't accumulate to a fashion disaster, and you let her, because all in all, you trust her. You may have some doubts when she swipes your black card without any hesitation at the mall, but as long as she's happy — she's stolen your heart, and now your shirts; maybe you'll have to let her steal your last name, too.
She says "I love you" when she accompanies you to one of your favorite Tokyo Spirits games, despite having little interest in soccer and even less interest in your favorite team. She sometimes cheers for Big Osaka or, even worse, goddamn Ryusuke Higo, when the two big teams go head to head, but you'll let it slide because of how happy she looks enjoying something you love, and sometimes you miss an impressive goal or two on the field because you were too busy staring at her.
She says "I love you" when she assists you at yet another crime scene you run into casually. When Inspector Megure and the other police officers arrive, they shake their heads at how they always seem to run into the two of you and they marvel at your impeccable cooperation. She follows your train of thought and oftentimes finishes your thought for you with a confident smile on her face when she knows what you're about to say. Maybe calling yourself the Sherlock Holmes of Japan was a little narcissistic, but either way, she was your Watson, without a doubt.
She says "I love you" when she cries on your shoulder in front of her sister's grave. Every year you visit Akemi, you've got the gloomiest mood, because you have no idea how much worse it must be for her. You wish that Akemi could still be here to accompany her, because she's not as strong as she says she is, and you know that no matter how hard you try to protect her from everything, you can't be perfect, so all you can do is hug her tighter and hope for the best for her.
She says "I love you" when she opens up to you about her insecurities, about her past, and about what she feared the most. Late night, messy hair, she tells you about the Black Organization, about Gin and Vodka, about Vermouth, about her parents, and about the pill that changed your lives forever. She talks about how little she used to feel like her life was worth and how alone she used to be with a rueful smile, but she rolls her eyes when you try to comfort her. She tells you about these things because she trusts you, just like you trust her, and you can't help but think that even in times like these, she was heartbreakingly beautiful.
He says "I love you" when he shows up at your workplace as you were about to leave, handing you the biggest bouquet of flowers you've ever seen. You've never even imagined it was possible to find a vast variety and large quantity of flowers before, but hey, he was Shinichi Kudo, for God's sake, and he was ridiculously stupid for someone so smart. Sighing at the excited voices of your coworkers, you take the bouquet from him as you tell him that you didn't need flowers, much less this many of them. You ignore his childish grin as you take his hand and head back home. Just how immature could he get?
He says "I love you" when he hides your supply of black coffee from you. You could barely get any work done without it, so you constantly pester him to restock your collection. He gives in once or twice, but he always tells you that it's unhealthy as hell and if you kept this up, he'd be angry. It's inconvenient, it really is, but he cared about you, so you could only sigh and give in. Besides, he was always there when you fell asleep before your laptop, and he'd always carry you to the bed and cuddle up next to you.
He says "I love you" when he excitedly talks about the newest mystery he's seen or the most recent Holmes novel he's revisited. He knows you're listening, and you are, and he's happy that you understand him. You can't help but smile a little when he does so; to the entire world, he may be the Savior of the Police Force, but when he's in front of you, he's just a boy who loves detectives and mysteries, and you're glad that you're the only one who gets to see this side of him.
He says "I love you" when he takes you to visit his parents. Yusaku-san's novels have always been interesting to you, and because of his sharp logic, dinner conversations have almost always turned into intellectual debates. Yukiko-san, however, was bubbly and energetic; she treated you like her own daughter, spoiling you with trips to the mall and showering you with affection. The Kudos were nothing like your parents, but you loved them nevertheless, and you knew that he wanted to give you the most warmth possible.
He says "I love you" when he actively communicates his thoughts to you. You know that he's probably the most stubborn person ever and that he's never been the best at understanding girls, and you'll admit that being with him has made you softer than usual, so you could seem a little unreasonable at times, but he puts down his pride for you and only you. Arguments never end in too much destruction, because he values your input and he cares about you.
He says "I love you" when he tries to protect you from anything and everything. You point out that you are neither a delicate angel nor a damsel in distress and that you were fine on your own. But he takes your hand and looks you in the eye to tell you that "he's promised, and he'll always keep that promise by being here," and he shoots you that smile of his that makes you weak. In the end, you give in and let him to a certain extent, because he knows you're not as strong as you say you are, but you love him too much to let him carry that burden of being your knight alone.
They say "I love you" through every gaze, every touch, and every smile — perhaps there was no need for words to communicate the overflowing love they had for each other, but they did so regardless, wholeheartedly.