Reviewing why this series fails requires major spoilers for later chapters, since poorly written plot twists make the story implausible and the writing lazy.
A classic tale of opposites attract, Sumire wants to fall in love for the first time, and is seated in her high school class near the mysterious adult Leo with dangerous rumors swirling about him. In his 20s and seemingly born into a wealthy family, Leo's seemingly violent and dangerous past and intimidating facial expressions scares those around him, until Sumire discovers a soft side to him. He, however, tries to distance himself from her, worried about his past harming her. And
his past definitely haunts him and harms those around him: random people attack him and threaten those around him, due to his criminal past. Although born into a wealthy family, he works part-time in a bar seemingly due to his alienation from his wealthy grandfather.
There's redeeming qualities, at least early: Sumire falls for him pretty quickly, and unlike many stories, takes the initiative. Leo's backstory actually threatens her, giving him a legitimate reason to be fearful of getting close to her. He's flirty and smooth in his interactions with her and he deeply cares for his younger sister, softening our initial impression of him.
But the story completely falls off the rails. Leo comes clean about his past and why he's been so distant to her: he kidnapped her as a child and nearly got her murdered. So what excellent foreshadowing did we get for this monumental reveal? Well, nothing, asides from the cheapest plot devices to justify it. Although was likely 16 when he kidnaps her (making her 9), she has no recollection of it at first. Selective amnesia, my old friend. And we see him "save the cat" to show us he's no longer a bad guy at heart. The only clues we had previously were that she has a curfew and that she feels guilty for causing her parents divorce the revealed the chapter before we learn Leo kidnapped her. Truly, excellence in writing.
It gets worse. Leo's been secretly sending her dad money daily for the past 7 years, remorseful about his actions. When is this revealed? Well, of course moments after we learn he kidnapped her. Does the dad know who her love interest is? Of course not. And the "small sum" of money he's sending daily from his part-time job? ~$150, so he's making ~$55,000 per year working part-time in a bar.
The age gap romance actually gets worse with later developments: rather than him be 20 and her 17, she's 16 and he's 23. He also fell in love with her and was watching her from afar when she was 15 and just starting high school. If you're weary of age gap romances, their relationship gets downright creepy later on.
And finally, the characters are weak and barely resemble real people. In romances and especially dramas, this is one of the few critical parts of a story: if the characters aren't relatable or realistic, the story falls apart. And our cast is anything but. When another student has a crush on Sumire and Leo threatens him, he starts actively helping Leo and Sumire get together. Why? He wants to become the third listed character if this was a play. He swoons over their interactions, and helps them find scenarios to be alone together. Is he jealous or wearing a thinly concealed mask? Of course not. Upon learning Leo's been sending money daily, Sumire rushes to mend things with him. Kidnapping and attempted murder be damned, nothing can stop true love.
Simply put, the terrible writing and mediocre characters simply aren't worth your time. The art isn't particularly good, and is fairly average fair. What starts off on a promising foot quickly devolves into a mess of a story.