When many people said "eugh, Ruby need to get over her crush on Gorou who now is her own twin", I'm busy thinking about Gorou's, or, Aqua's own "crush".
Gorou, first of all, was not a fan of idol before he met Sarina. And later on he became Ai's biggest fan, especially after he lost Sarina. What I see the most from this is that Gorou is in his own way, a "people pleaser", especially to his precious people. It came with his job as a doctor and his loneliness. And it also can be his own brand of kindness. And maybe being Ai's biggest fan was his coping mechanism.
When he met Sarina, his world has changed bit by bit. He never thought of his own life as unfortunate, but then there was a little girl who lost her own self more and more so fast. And there was no one to support her. Yet she was so radiant that it left a big impression in Gorou's life. The way she still had the capacity to love her idol, Ai, as deep as that was so dazzling. And tho I don't doubt Gorou's grandma's love for him, Sarina might be the first to show and gave him love so straightforwardly. And being emotionally stunted man he was, didn't know what he felt for her but Sarina IS the biggest presence in his heart, even following onto the reincarnation.
Now, I know that Aqua loves Ai so deeply. But for what I see, when he was Gorou, he became a fan of Ai more for Sarina's sake, at first. To give her a friend who share the same interest and all. But later on he became a genuine fan. BUT, remember how he sometimes saw Sarina's shadow in Ruby?? It might be similar with that. He saw Sarina's shadow in Ai, and now vice versa. Even Director Gotanda saw Ai in Ruby.
And above all, Gorou/Aqua can't differ his own feeling of 'love' he doesn't know if he was 'in love' with Ai or not, and he take interest on Akane when she start to mimic Ai and can be used. And there's Kana, who he can banter easily but in the end he use her for his own purpose. That doesn't mean he doesn't care for them, it's the opposite. Despite how crass he used them, he does care deeply and doesn't want them to get hurt. Which fail spectacularly.
But Ruby, Ruby is that one existence he won't ever use for his purpose. And he will go to hell and back to keep her 'radiance'. It's the same with how he treated Sarina and Ai, like they're the precious goodness in this bleak world and have their own circle in his heart. He knew of their flaws, yet still treat them like they had no fault.
He laughed meanly at how naive Kana is, how easily she swayed by the boy she likes. But he tries so hard to preserve Ruby's innocence, or what left of it despite knowing how hard her life was/is. Even when he listen Ruby said of how broken she was, she's still the most radiant.
I do ship AquaRuby, but I won't said that Aqua doesn't have a "crush" on Ai, Akane, Kana. Like I said, Gorou/Aqua is emotionally stunted about his "love". He cares deeply, craves for love, but doesn't know how to receive and reciprocate. His "love" is of self-sacrifice kind of way. He was fucked up when still as Gorou, losing Sarina when she said her confession, and then losing Ai as Aqua in almost the same way. Rather than focusing on his own "love" and discovering it, he's more desperate on not losing anyone else. That said, he wants to finally have his happiness. But as long as the biggest threat (aka Kamiki Hikaru) still running around, his precious people is in a big potential danger, especially his sister Ruby, his idol's daughter who's so alike with their mother, and the reincarnated "Sarina-chan". He feels he owes them (Sarina-chan & Ai) for loving him.
Why he feels like he owe them? From my own experience and from what I know of Gorou, people like us need others (precious people) approval and love to feel like we have a living purpose. Even then, we felt that we need to do more to not lose them. Loneliness is a scary thing. Maybe that's why he attracts other lonely people (the girls). He show them the support they want, and sometimes show some empathy.
And for him, Sarina-chan is the loneliest. She suffered so greatly even without that loneliness. And then he met Tendouji Marina as Aqua, and once again reminded how much Sarina had suffered and abandoned. Even then she still waited for her mother, wished for her parents health, loving Ai and Gorou, smiling and dreaming, etc.
And well, yeah, thats why I think the one Gorou/Aqua loves the most is Sarina/Ruby. I don't know what kind of love it is, and don't know who will end up with Aqua, but for now Sarina/Ruby is his number one girl.