This one fits both Thorn (from Alone, anyway) and Brethren; but I picked Brethren because Thorn changes his attitude toward the end. 

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I'm a savage, I'm a fighter: This line is very true. Brethren is a savage to the whole world, rampaging against innocent people and having his fun with them. He's terrorizing the world, and he's enjoying it. Ever since he was put into the Abyss, he was wanting to break free again - because Earth is so fun to mess with. 

All I feel's the adrenaline: Brethren loves the rush of battle, of toying with his playthings, of angering or frightening his victims. He is a torturous, terrible monster that isn't even human - and no one can comprehend him through their fear of him.

And when the pain begins to set: Brethren has fought against Shintaro before. He has been in on Earth before, and though it isn't mentioned when, Shintaro was old enough to battle against his father using Light Magic. Eventually, Shintaro managed to shove him into the Abyss, the eternal realm of darkness. 

Long live the champion: Though most Darks only serve The Dark Lord because of their fear of him, there are some that serve him for the mere pleasure of it. They too fear him, but they love the rush of battle and toying with humans. They chose their side with The Dark Lord, and they're cheering him on, basically. 

Call me names, call me reckless: When Brethren is in a smug or good enough mood, he doesn't care what people call him. If he's going through a rush of adrenaline, he usually spits insults right back with an arrogant demeanor. Unless it's coming from his own followers, he actually enjoys the names that people call him. He knows that he's murderous, reckless, and terrible; but he does nothing to fix it. He enjoys it. 

Filled with fury, I'm relentless: Shintaro knows that when Brethren is angry, there is nothing more terrifying. Brethren in general is a terrible man, but when he's angry... The victim of the fury would probably rather be dead than to experience what Brethren has ready for them. 

When everyone around is breaking, I'm a stone in the ground: Brethren knows the power of his son. He is well aware that Shintaro can take out many of his Darks, but he knows that Shintaro will have a hard time defeating him. He will not back down - Earth will be his. 

~

Brethren is determined that he's going to be the champion, because he knows that there is only one of Shintaro - and many Darks.