Effect on the Main Plot
While Fuko helped bring Nagisa and Tomoya closer together, Kotomi didn't really do that. Sure, the two helped out with planning for Kotomi's birthday, but that didn't bring them too much closer together. Instead, Kotomi helped build, or, rather, reveal, Tomoya's character.
Along with the building of Nagisa and Tomoya's relationship, the main plot of Clannad also involves the exploration and development of both of their characters. Kotomi's arc is an example of the exploration of Tomoya's character. The arc starts to show us Tomoya's childhood: he was a kid who ran around chasing butterflies, and he ran across this strange violin-playing girl one day. We see how easy it is for Tomoya to make friends (which is a trait he carries, at least to an extent, in the present day as well). We see how he at first gives up on things when facing challenges (like being ridiculed for wanting to go to Kotomi's party), but in the end he gets past them (not only going to her house, but helping save her from a fire as well).
These aspects are shown in the present day too: he perseveres in landscaping her yard, even when it seems the task will be impossible. This is also tied into Tomoya's loyalty and devotion to his friends, shown in both his childhood and the present day. In the end, then,Kotomi's arc helps reveal Tomoya's character: the childhood flashbacks show us that many of his character traits have been with him his whole life. (By the way, the fact that they were childhood friends is why Kotomi was justified in calling him "Tomoya-kun"...even if he didn't remember this fact.)
The second area where Kotomi impacts the plot is in the realm of the ending and the invisible world. Though we first saw one of the lights during Fuko's arc, they become much more obvious during Kotomi's. When she opens the briefcase from her parents and realizes just how much they loved her, the air is filled with lights (again, the lights will be explained in detail in a later post).
Besides the lights, we also gets hints into the invisible world through Kotomi's story. In her childhood, it is shown that her parents were researching a supposed "hidden world" that was different from our world yet somehow integrally connected. Her father thinks of the world as consisting of countless invisible harps that when sounded in their individual sounds create a single melody, which explains the beauty of the world (hence Kotomi's name, according to her father: "koto" is the Japanese word for harp). Her father's words (as relayed by Kotomi) also help explain the "magical" nature of the show: "Those who seek out the truth must not be arrogant. You must not laugh at miracles just because they cannot be explained scientifically. You must not turn away from the beauty of this world." Her mother's words, on the other hand, express the theme of family better: they will be explained below.
At the New Year's party in After Story (a very thematically interesting scene), Kotomi tries to explain the "hidden world" theory using quantum mechanics and theoretical physics. When that fails, she resorts to the simple explanation that it is somehow connected to our world, even though her parents thought it was a world we could neither see nor feel. "Time, space, and people's consciousness interact in mysterious ways to change both worlds or create new worlds," she says. The invisible world could interact with our world: it's truly unknown, because the world has many mysterious aspects. The "many worlds" theory also comes up in this discussion.
Though all the theoretical discussion is interesting, I think the most important aspect of this is that when Kotomi is talking about the hidden world in After Story, Tomoya has flashes of memory involving Nagisa's play (whose plot is based on the invisible world) and the invisible world itself. That solidifies it: the Ichinoses' research does investigate the invisible world we've been seeing.
To summarize, then, Kotomi affects the main plot by revealing more of Tomoya's character and by offering insights into the show's magical nature and the nature of the invisible world.
Again, family is the main theme of Clannad, and it is very present in Kotomi's arc. Her arc truly revolves around the effect that the loss of her parents (and, in a different way, her childhood friend) had on her. It is very apparent that she loved her parents deeply, and they loved her. Her mother's words to her as a child resound as a statement of the importance of family: “Remember, what’s important is always very simple…You are Mother and Father’s most cherished treasure.” That familial love is important in everyone's life; without that love, it can be hard to truly become a mature person. Maybe that's one why Kotomi is such a reserved person: the loss of her parents. In her childhood, she has a good friend that she plays with, and her violin skills are actually pleasing to the ear. After the death of her parents, though, her entire life becomes devoted to studying so she can follow her parents' footsteps and collecting every reference to them ever written. She loves her parents to the point of obsession, and their loss truly devastated her. In the end, familial love is important. Without it, our development into mature persons is slowed, if not stopped altogether.
As mentioned in the previous paragraph, Kotomi loves her parents to the point of obsession. This brings up the theme of perseverance. Kotomi's obsession with her parents is partly due to true familial love and partly due to an inability to get over the tragedy of their death. That is why she has her father's study plastered with clippings of newspapers and books that mention them (hence her cutting of books in the library and bookstore). This started with just clippings related to their deaths. The closed off room full of clippings and the spot where she burned the catalog represent her mental state in a way: just as she locks herself into that room, she has also locked herself into the tragedy of her past. Thanks to the efforts of her new friends and the reception of her parents' briefcase, though, Kotomi is able to leave that room, both physically and mentally. I've always loved the words of her parents' letter to her: "The world is beautiful. Even if you're full of tears and sadness, open your eyes. Do what you want to do. Be what you want to be. Find friends. Don't be in a hurry to grow up. Take your time." These words are a stellar example of perseverance: even when tragedy is all you know, open your eyes and move ahead. The world is beautiful.
There's some self-sacrifice present too, in that Kotomi's friends gave up lots of their time in helping make her party the best it could be, and her parents gave up the sum of their entire careers (the manuscript) in order to send one last message to their daughter.
The supernatural is present here in terms of the lights that appear when Kotomi receives her briefcase and the investigations into the invisible world.
Another theme in Clannad is the influence of the past on the present. To be more specific, you could say the influence of childhood. The tragedy in Kotomi's childhood kept her imprisoned for many years, yet there were still joyous aspects of her childhood that continued into her adult life: for instance, her love of the violin and her dedication to her parent's work. We also see that aspects of Tomoya's childhood personality are still present in his older self (see the "Effect on the Main Plot" section for details). I think this idea can be summed up in the sentence from Robert F. Young's story "The Dandelion Girl" that Kotomi quotes: "The day before yesterday, I saw a rabbit. Yesterday, a deer. Today, you." This sentence is what clicks Tomoya's mind into remembering his childhood friendship with Kotomi.
Kotomi's arc always struck a chord with me. Besides the main storyline of Nagisa and Tomoya, her arc is my favorite. I've always felt like I connected with Kotomi. Maybe it's because I used to be an overly quiet, studious person, maybe it's because I understand what it's like to lose a parent at a young age. Either way, I've always felt her story was powerful. I've especially always loved her parents' words: they've inspired me since I first heard them (which is why I quoted them in full above). The comparison of the world to a flowing melody, the beauty of the world, the necessity of family, friends, and perseverance...it's all extremely beautiful to me.
I also think there's an implicit theme in here as well: the transience of friendship. While friendships can last for a lifetime, most of them pass away; at the very least, they grow colder in intensity. Sometimes this happens because of tension or conflict, and sometimes it's just a gradual passing away. I think both of these can be seen in Kotomi. First of all, her friendship with Tomoya came to an abrupt halt after her parents' death psychologically wrecked her. That's an end due to tension (though not tension between the two of them). Second of all, after their friendship is restarted in high school, during the arc, it slowly drifts apart. Kotomi very much becomes a background character at the conclusion of her arc. Though she retains her individuality, the intensity of her friendship with Tomoya dissipates. Some of my friends have complained about the fact that Kotomi disappears into the background after her arc, but I think it's a sadly realistic thing: I've had that happen to me many times. I went through 5 or so groups of good friends in my first year of college. I'd have a great, pretty intense friendship with a group for a while, and then we'd slowly drift apart, and I'd eventually merge into another group, and the process would repeat. So I think Kotomi's arc is realistic in that way.
Overall, I do believe that I love Kotomi's arc, and I think it's a powerful tale that adds to Clannad's themes and helps reveal Tomoya's character. So now we end with a look at the final glimpse of Kotomi in the After Story montage during "Chiisana Te No Hira," as she drives off into the (most likely American) sunset...
Thank you for reading. God Bless, and peace.
Nota Bene: All clips are from the Clannad Central YouTube channel run by the Clannad (クラナド/Kuranado) fan page on Facebook. All character themes and other music from the show can also be found on said fan page, in the music player. My gratitude to them and all the work they do.