RaikouGau/GauRaikou Manifesto
In chapter 23, Yukimi states that Raikou really seems to like Gau because he is always praising Gau. Kairoushuu never seems to spend a huge amount of time together, nor are they super close, except in the case of Raikou and Gau, Miharu and Yoite, and somewhat Yukimi and Yoite, though in a different way. For Yukimi to make this type of observation must mean that Gau is quite close to Raikou.
"Can I become your strength? I've always admired you."
When Raikou is injured in a fight and Gau tries to help, only to get hurt, Raikou has a fairly extreme reaction. Perhaps he really does want to make up for his inability to save that girl, or perhaps he has spent the whole time caring about Gau the person, not what Gau represents.
Gau says he wants to help Raikou, to which Raikou only smiles.
Raikou feels incredible amounts of guilt due to the fact that he believes himself to be the cause of Gau's injury and mental anguish.
Raikou tells Gau that if Gau breaks the promise to tell anyone about the truth of the Shimizu clan, he will be forced to strike Gau down. Not that he will, but that he will be forced to, implying that he really doesn't want to. However, when he realizes that Gau has broken that promise as Raimei asks him to return Black Gamon in chapter 26, he does not act upon this knowledge. When Gau comes to receive his punishment for breaking the oath, Raikou is shocked and in despair; he probably didn't think Gau would do so, or he didn't think he would unintentionally and accidentally go through with his half of the promise to kill. He can't believe he has hurt someone close to him, but this time, the situation is not like hurting Raimei for an ultimate good; there's no reason to hurt Gau. For Gau to be willing to throw his life away, he must really believe in Raikou. Even if Gau's main reason was that he didn't like family fighting, to die just so somebody can have a peaceful life with his sibling is a tad out there unless there are serious feelings involved.
Raikou cries when he visits Gau in the hospital. He is not a stoic character, but he does not show copious emotions, either. Raikou has killed before and does it on a regular basis, but he does not show strong emotions because of that. He has struck a deadly blow with his own family sword to someone he cares for deeply, and his tears exist for a reason.
Miharu tells Raikou that Gau has told him something, to which Raikou responds "I envy you." Does he think that Miharu is trusted more by Gau, and that he wants to be the one to which Gau tells things? Or does he think that Gau has gone through with Raikou's wishes and left him, and he recalls other times when Gau did tell him things, if only some? Raikou also says that he has to get Kouga's scroll because he wants Gau to see him one more time. In all honesty, they both want to see each other.
"I have to suffer because I absolutely don't want to lose him."
"All I care about right now is my...friend."
Raikou cannot even come up with the proper word to describe Gau. Is it because he has hurt Gau so severely and now wonders if they can be friends, or is it something more? Either way, there is some type of intimate relationship.
Raikou also says that "Gau IS [the girl I couldn't save before]" and that "by saving Gau, I was saving my own weak, past self." Does this mean he doesn't care about Gau at all and only sees him as a replacement, and he must help Gau because he couldn't help that girl? This theory is disproved, though, given all the above evidence.
In chapter 47, Raikou is trying to turn Yukimi over to their side so that he doesn't have to kill Yukimi, but then Yukimi aims his gun at Gau and the fight is on. Raikou chases after him and swears that he will kill Yukimi. Why? His face is very serious and he's really outraged. Could he honestly become so angered over a threat to his plain old subordinate?
Gau's reaction to Raikou's question in chapter 48 of whether or not he wants to leave Kairoushuu is not met with much of a reaction, but his question of whether or not he wants to leave Raikou is. Raikou says he will be somewhere where some part of Gau can find him and Gau blushes hardcore, complete with sparkly eyes.
"If I let you leave, I cannot protect you from Kasa."
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~To be updated as more chapters are released and scanlated.
If anybody has anything else they'd like to add, do leave a comment and I can edit this. I know this is definitely not everything, and I left out a few small things that I thought weren't really necessary since there's already so many other events that mean the same thing.