Despite being much younger, Ganya's younger brother Kolya turns into Myshkin's BFF because he is so idealistic that Myshkin's perfection can't help but inspire him.
Sure, you're asking, if Myshkin is such a great Christ figure, then where are his disciples? Well, what he lacks in quantity, he more than makes up for in quality—after all, if you're going to only have one disciple to follow you around and learn what you're teaching, you could do a lot worse than Kolya. Like the novel's other young people (Ippolit and Aglaya, basically), Kolya seems to be pretty naïve and innocent when the novel opens. Well, obviously not as naïve as Myshkin, since that guy is just a walking blank slate, but still, pretty sheltered. How do we know? Well, for one thing, Kolya is still listening to the nihilist nonsense that Ippolit is peddling—and we all know how Dostoevsky feels about nihilism. (Spoiler alert—he's probably not a fan.)
But as he slowly starts to smell what the prince is cooking, Kolya's underlying caretaker traits come out: he is the only one who knows how to deal with his alcoholic dad. He not only sits up nights with the slowly dying Ippolit, but actually explains to everyone else that Ippolit's bitterness and rudeness might have something to do with the fact that he is terminally ill—and also that in some ways, Ippolit might really have a point. Finally, the only reason that Myshkin is at least transferred to the competent care of the Swiss clinic at the end of the novel is because of Kolya and his doings. Check out this scene, for example, when Kolya distills some meaning out from Ippolit's crazy confession, while at the same time trying to take the spotlight off of Ippolit's totally disastrous suicide attempt:
"Well," said Kolya, plunging in medias res, as he always did, "here's a go! What do you think of Ippolit now? Don't respect him any longer, eh? [The explanation and suicide attempt were a result of] Agitation—excitement—all that sort of thing—quite natural, too! […] "I'm awfully impressed, you know."
"Naturally, all this—"
"No, no, I mean with the 'explanation,' especially that part of it where he talks about Providence and a future life. There is a gigantic thought there."
The prince gazed affectionately at Kolya, who, of course, had come in solely for the purpose of talking about this "gigantic thought."
"But it is not any one particular thought, only; it is the general circumstances of the case. If Voltaire had written this now, or Rousseau, I should have just read it and thought it remarkable, but should not have been so impressed by it. But a man who knows for certain that he has but ten minutes to live and can talk like that—why—it's—it's pride, that is! It is really a most extraordinary, exalted assertion of personal dignity, it's—it's defiant! What a gigantic strength of will, eh?" (3.9.34-42)
It's interesting to think about Kolya being a kind of "disciple" for both Myshkin and Ippolit, both of whom could be said to do a bunch of preaching in the novel. What does he learn from one? And the other? Also, why do you think the novel's teenagers aren't ever given the chance to be sheltered and innocent? Is it because they would then affect how the prince comes off?