Kirishima Ayato
Ayato is Touka’s little brother, who shares similar traits and facial features with Touka, except that he’s much more violent. He cannot accept his father’s ideals of living peacefully with humans. He left the 20th Ward to join the Aogiri Tree.
Ayato appears to be stronger than Touka. In Tokyo Ghoul, he’s mastered his kagune to grow out both ‘wings’ when Touka only manages to grow out one. He’s also mocked his sister for being ‘weak’ numerous times, even attacked her and tore her kagune.
Ayato initially bears hatred towards Kaneki because of the resemblance of his personality to his father, Arata. When the Aogiri ‘kidnaps’ Kaneki, Ayato beats him and tells him to address him with honorifics (while Ayato is actually the younger one).
Kaneki is able to see through Ayato, that he attacks Touka out of love, in order to protect her (tough love, duh). Ayato keeps denying this, but then he is distraught and remembers a fragment of his memories of little Touka crying; her clothes ripped by the kagune that came out of her back—which indicates that he does care about her. Despite knowing this, Kaneki doesn’t hesitate to half-kill him, breaking half of his total bones in cold blood while reciting the names and amounts of the bones. Kaneki considers this act as a mercy, because Touka loves her only brother and she will be upset if he kills Ayato.
Of course, after some time, Ayato is able to heal—the perks of being a ghoul.
In the manga, Ayato isn’t shown to properly meet Touka afterwards, but he takes up her identity as “Rabbit”. It is possible that he’s doing it to avoid herself from being hunted by the investigators.
Several years past their fight in the 11th ward, Ayato has grown to be a good-looking man taller and stronger, even the CCG has recognized him as an SS-rate ghoul. He’s able to manipulate his ukaku to take a form of koukaku, which allows him to fight better in close-range combat. He unexpectedly reunites with his sister when they’re in Cochlea, while trying to save Hinami. Touka and Yomo save him while being attacked by the investigators. Ayato is upset by their presence, but Touka says not to worry about her.
Ayato: “Do you have any idea where we are?! At this rate, you’ll…”
Touka: […] “Ayato, you don’t need to keep overreaching. You’ve grown tall enough already. Rely on others for a change. Stop worrying about me, and go protect Hinami.”
Some of us are probably longing for Kirishima drama, at least for Ayato to apologize to his sister. But I think both Ayato and Touka have problems expressing their feelings, especially towards each other. Touka probably understands that Ayato won’t apologize to her for all the things he’s done, and she probably doesn’t need his apology anyway. For the Kirishimas, action speaks louder than words.
Ayato then tells her to do whatever she wants, and mocks her by asking if her body’s grown numb (since she seems to have a rather quiet life after Anteiku), to which Touka responds Who do you think I am, you shitty kid?
They finally rekindle their relationship, and afterwards Touka and Ayato stay close to each other.
When Kaneki then enters the battle to protect Yomo from Kishou Arima, he tells Ayato to escape with others through the underground channel. Regardless of their past, Ayato doesn’t disobey Kaneki—he even worries about his sister, aware of her feelings toward Kaneki.
Probably, Ayato has realized about Touka’s affection ever since she went to the 11th ward to save Kaneki. I think he can understand why Touka falls for him—afterall, he’s mentioned that Kaneki resembles their father, and Touka has always admired Arata. I wonder how it feels for him though; the man his sister’s in love with is the very man who has tortured him half-dead. It must be difficult for him, but on the brighter side, he probably knows that it shows how much Kaneki treasures her.
Kaneki: “We’ll infiltrate the lab with only a select few. People that can take a hit… Ayato-kun.”
Ayato: “Why am I the first person of sort that comes to mind? Geez…”
After GOAT’s inauguration, Ayato joins Kaneki’s organization. When Kaneki chooses Ayato to come with him in a mission to retrieve the RC suppressor, Ayato protests, but still carries out the deed anyway. It is actually entertaining to see this conversation, because Kaneki still remembers that he’s done quite a damage to the poor boy, and that he’s strong. On the other side, Ayato probably doesn’t want to recall the memory. It is clear that Kaneki trusts him, though.
During the infiltration, Ayato realizes that Kaneki has a problem as a leader. Their mission is to retrieve the RC suppressor, which they have done, but Kaneki is distracted to see Amon going berserk, therefore hesitates to leave. Ayato tries to knock some sense into him, that he needs to make decision and let go of some things, not be swayed by emotions.
“I hate to admit it, but... even I ended up having to rely on you. [...] Even my sister… is waiting for you. And you’re ready to throw it all away for your emotions?! If that’s true... then you really are a half-assed bastard.”
Although Ayato is younger than Kaneki, he’s got more experience than him in fighting as a ghoul and a leader. He’s more tactical and aware of responsibilities. Kaneki finally agrees to leave when Seidou offers to take care of Amon, together with Kurona.
Kaneki marries Touka without her brother’s consent, because Ayato’s currently on a mission to explore the deeper part of the 24th ward. By the time Ayato returns and reunites with the rest of GOAT, Kaneki has already turned into a gigantic creature lurking in Tokyo. When he arrives, everyone else is talking about their wish to save Kaneki, and what they want to tell him, and then Touka and Tsukiyama brings up baby names topic. But when Ayato joins the conversation, he just asks them what exactly they were planning (to the monster), not reacting to the baby topic.
I honestly have expected a more dramatic reaction coming from Ayato to learn that his sister has smashed with the Kaneki—unfortunately, there wasn’t, haha. I guess it means that he’s totally accepted Kaneki. Like Nishiki, he probably expects that this is bound to happen one way or another, considering Touka’s unwavering affection since 4 years prior. Ayato even brings back three underground children with him. I guess the love for children is carried out within Kirishima genes...?