Prepare yourself.
Takashi is a character introduced in Matrix. He is Hiiro and Rinne's little cousin and stole Hiiro's childhood 2D live model because Happy Elements can't be fucked to give their NPCs designs even when they dominate and control the story. Takashi's mother is someone named Akan-P who worked with Rinne when he first joined the idol industry as a teenager. Her lore isn't really important as she's another useless shit NPC who fucked up the Amagis' story, but she ultimately lands in the Amagi Village and shacks up with Rinne and Hiiro's uncle. Amagi Papa really likes Takashi apparently and wants him to be the next chief. Even though you know. Rinne is there. In addition to being unnecessary, useless and a distraction from the main characters in Matrix, Takashi creates a serious plot hole. It calls to question Hiiro's entire journey to the city and mission to bring back Rinne. Fans have theorized this is a set-up to make it so the writers don't have to discuss the Amagi Village anymore. If you aren't aware, Matrix essentially existed solely as a shitty "We're sorry Ainu people." By "response to the Main Story," Happy Elements meant an (inadequate) response to the criticism it received for its racism. Matrix retconned the concept of the Amagi Village and changed it to some Ainu futurism utopia. As opposed to just, letting them be a normal community who is modern and has the ability to use the same technology as the rest of the country. Weird overcompensation that sent a bad message about what Indigenous people have to do in order to deserve respect. Anyway, if Takashi gets raised to be the next chief, it diminishes the importance of the village to the story and can be an excuse to not come back to it.

Hiiro would enjoy all bonding activity regardless of what music it is, because he's just like that and they were apart for so long. It must make him very happy when he is able to spend time with Rinne these days. That said, Rinne pushes Hiiro to be more independent so he might encourage Hiiro to make the music selection. I think Hiiro would like to listen to Rinne's music, but Rinne would be embarrassed by that lol. You know, Hiiro's probably gotten to know older idol music too from Aira. This is good Nii-san Bonding Material. I do not picture the characters of Enstars as existing in the real world with real world figures, so I can't really imagine them listening to any actual artists. I think it's different from other parts of real pop culture existing in the Enstarsverse. Mainly, I don't want to think they inhabit the same world as me because it feels weird to me. Rinne and Hiiro listening to music from artists who are active in the real world feels uncomfortable to me somehow. I like put a certain distance between myself and the characters.

Well according to Niki in Ariadne, it seems Rinne has some sort of thing about sadomasochism. He also got a kick out of humiliating Niki by trying to get him to publicly strip in the original script of Night Club. Got flirty again in an Idol Story with Niki where he straight up said he was getting turned on by looking at Niki's nape. In that regard, Rinne must be one of the few Enstars characters where we factually know one of his fetishes because he has announced it lol. The back of the neck is a vulnerable spot. Furthermore, Rinne is often depicted in artwork as looking down on the viewer with a pleased expression. Don't forget he was the leader of the Leather Harness Shuffle Unit. I think they might be trying to tell us something about Rinne! (he's a sadistic dom, or Do-S as we say in Japanese lol)
Some of this stuff can be seen as part of Rinne's "Bad Boy" shtick but we've seen this behavior typically pops up when he's with Niki, who knows the real him and seems to believe that Rinne is a sexual sadist. So I'd say Rinne is quite Freaky.
This is all I have to say on the subject.

First, I don't think we should talk about Shu as though he has agency. Shu is at the mercy of his writers and will say whatever they think is entertaining. Distaste towards Indigenous people is a pattern in Shu's writing. Not because the writers think it's a character flaw to be taken seriously and addressed respectfully with him hopefully growing past that mindset, but because some people at Happy Elements and some of the fandom get a kick out of racism. As a result, Shu has unfortunately become a tool through which offensive jokes are made. In Snow Globe, we hear from Mika that Shu looks down on Indigenous cultures. I don't think he would make that up. Mika's shock at Shu's "homage" to Indigenous people and assertion that Shu thinks poorly of them is a reflection of his observations as someone close with Shu. In Shinsekai, Shu has the order to not insult the locals in Touhoku. There's many Japanese people who look down on Touhoku natives and perceive us as uneducated and unrefined. This comes back to the fact Touhoku has historically been home to Emishi and Ainu people. Do you know where the word "shogun" comes from? It's shorthand for seiitaishougun, which means "commander-in-chief of the expeditionary force against the barbarians." Who were the barbarians? The Indigenous people in North-Eastern Japan, Touhoku included. Japanese people have long looked down this part of the archipelago because of this history, and this attitude holds true for many (but not all) today. Our dialects that share features with the Ainu language are frequently mocked and associated with being "backwards" and "uncivil." Some might even say, barbaric? Shu apparently thinks poorly of people from Touhoku. All of this adds up. Even if you dismiss that he directs a slur at Rinne because he uses it non-exclusively (though do remember this word has a loaded history and is a favored insult by racists, and we know there's some of those at Happy Elements), these other glimpses into his world view exist. This is how Shu is characterized. It's unpleasant but it's part of who he is, just like everything else the writers have done with him.