I really like this song in general, but there are also so many reasons that this fits Shintaro. 

All the words you used to say: Shintaro's reliving the past, where his father would yell at him and threaten him. 

Tried to keep me down: Brethren would always downgrade Shintaro, and the older Shintaro got, the more he wanted to fight. 

Tell me when you kicked me did you ever think that I would get up: Brethren did kick Shintaro on several occasions; and although this is just a metaphor, Shintaro is basically saying "There's no way you're keeping me down for long." It makes me think of Found, when Shintaro is determined that he can defeat Brethren this time. 

Can you hear me now so loudly?: This also makes me think of Found, where Shintaro's like "I'm not hiding anymore. I'm not going to cower away from you like I did all those years. You're going to hear me this time, Brethren." 

Looking up at what I've become: Shintaro has become who he is because of his father. That isn't to say that Shintaro is a bad person like Brethren - but that Shintaro is fighting him, fighting Darkness, and rebelling against his father's wishes because of Brethren. 

Tried to find the light between your shadows, but it always seemed to fade: I really like this line. Although it isn't mentioned, I headcanon that Shintaro - on a few occasions - really tried to give his dad a chance at being his father. But no matter how hard Shintaro looked, no matter how many times Brethren was in a good mood... The Dark Lord was and is not a good person. 

It took some time for me to learn to let go: Shintaro really did want to give Brethren a second chance, a third chance, and possibly even a fourth; but Brethren was nothing like the father Shintaro should have had. 

But I grew stronger from the pain: This was when Shintaro was young, and he didn't really know that Brethren's actions were those of a terrible, abusive father. That was what Shintaro grew up with, and he was used to it. But the older Shintaro got, the more he realized that this was not how a father should be. Shintaro let go of the desperate attempt to give his father redemption; and although it hurt him for a time, he grew to hate his father. 

~

You guys may have heard this song before, and the lyrics didn't mean much to you (or maybe the did, I wouldn't know.), but hopefully now they help you understand how great this song is for Shintaro! I really think this suits him through and through. He's boasting to his father that he's not going to give in, not ever again. 

Brethren is going down.