Teru is just a toady, Hoshizake has always stood up for Tsuyoshi and "using him as a weapon" means giving him a route back into society. Teru encourages Tsuyoshi's worst instincts like his moronic idea to start a yakuza group to overthrow the government.
Teru also intentionally undermined Tsuyoshi's relationship with his GF which nearly drove Tsuyoshi to take his own life.
Teru isn't a toady. If he was then he wouldn't have been so mad about Tsuyoshi when he was in his playboy act nor would he try to calm down Tsu whenever his emotions got the better of him. (See this hug and him trying to drown himself).
The yakuza stuff happened because Teru, Tsuyoshi and the others had no other legitimate way to get back into regular society. Remember, it had been months since the whole coup thing that destroyed the old shadow government. Tsu was stuck doing convenience store work because he was expelled due to his ties to the former shadow government yet he was STILL unpersoned even though Aino should have had the power to over turn that.
On the girlfriend situation, Teru did do Tsu wrong there. However, A) the relationship was ultimately screwed because Tsu was unpersoned and B)most of the shit was caused by the little brother being a shit.
Teru operates out of fear and convenience of aiding MC, not out of true care. He knows what he can bring for him, Aino at least will be forthcoming.
Teru doesn't operate on fear nor convience. If he did then he would have abandoned Tsu much, MUCH earlier. Rather, Teru operates on a zealous devotion to Tsu as he views him as a god.
Aino is a dense idiot. He operates on a act first, think second montra. Sure he likes Tsu, but because Tsu isn't currently there, he doesn't go out of his way to help him.
Teru is the type of guy who'd move mountains for you when it would have been better to go a different direction. Aino is the type of guy to call you to warn you about the mountains after you've already psssed them.