That Summer should be read with no expectations. Yes, it is considered both "shoujo" and "shounen-ai"; but, more than those confining categories, it's very much more a slice-of-life, coming of age drama between a host of characters whose friendships and relationships (platonic and otherwise) develop in the midst of the one summer before they choose their paths.
Kim Jumi, the main lead and a female character, is wonderful. Full stop. For a series that starts with the relationship between two guys, Jumi is a breath of fresh air for those of us used to "useless," "distracting," "mean," or just plainly not well-written female characters (especially with romantic male leads). She is realistically well-written, with flaws of her own and strengths of her own. She finds herself, over the course of the series, learns who she is and grows from that. She empathizes with her friends and the two male leads, and helps those around her as well while still being far from perfect in doing so. She also places importance on her relationships with her friends, which is super nice, and reacts to them the same way any high schooler would when they're troubled with romance, fights between friends, and more. It's all her qualities and weaknesses that make her so likeable, and looking back at high school as a time that has already passed for me, I can see a lot of myself in the Jumi as a student. So, from that, she diverges from either "shoujo" or "shounen-ai" stereotypes, and becomes a person of her own.
The two male leads, Hee Bum and Yoo Won, are also something very different than what is expected in conventional shoujo or shounen-ai. Hee Bum can be sarcastic and feisty, but also sentimental, anxious, and someone who finds it hard sometimes to express himself, especially to those important to him. Yoo Won is riddled with a handful of mysteries and a half; he clearly cares about the people around him, but he also does not reveal much and prefers to be alone despite being well-known and admired. Their relationship with each other and with Jumi is much more complex than just a love triangle (and in fact, there are none of those in here, really) or two love interests vying for the girl/guy. All three protagonists become friends, but each holds separate views of each other and themselves, and that difference in relationships and relating to one another is well-developed and explored in That Summer.
There are also other characters, given their own pace and time and importance, and though they seem minor they also play important roles in the lives of Jumi, Yoo Won, and Hee Bum, and thus an important part in this story as well.
That Summer is chock-full of small events in life and big ones that ultimately make up what it really feels like to be a high school student in the summer before you "have to" move on with your life and begin a new stage. Though every chapter has a new and suspenseful revelation, and each reveal keeps you guessing at where the story will go, it's amazing because it feels just like life at that age; you will never know where it will go from there. It's full of nostalgic moments (for those old enough to have left high school behind), with fights and breakups and jealousy but also in-jokes, sweet romances, a world too big to truly understand just yet, and equal worry about both your future career and the person sitting beside you as you grab lunch together or meet each other on a bench outside at 3 a.m. to talk about everything and nothing at all. That Summer really, in the end, talks about "that summer" that everyone still remembers years on from now, and I'm sure I'll be remembering this story years from now as well.