[ insert fancy headcanon icon here ]
it's hinted pretty heavily in threads that i've done in the past ( and definitely hinted heavily in drabbles i've written on takeshi AND takeo's lives ) but for all of those who aren't exactly sure what takeshi's ' job ' is and why it's so dangerous.
long story short, he's in a gang. with seunghyun being the leader of said gang. takeshi will vehemently refuse to acknowledge the fact that he's in a gang because he's been manipulated by seunghyun for so long that he'll just turn and tell people that the gang is his family -- the only family aside from takeo that he's ever known.
now, takeshi's not stupid, he knows that what his family does his wrong, he knows he wants out. but seunghyun took him and takeo in from a pretty young age, housed them and fed them and kept them alive ( though his methods are a little.... unorthodox ) and there's a strong sense of loyalty towards seunghyun because of that.
that's why takeshi feels like he can't leave. any blackmailing on seunghyun's part is just the icing on the cake, that little bit extra just to really drive that idea into takeshi's mind that he can't leave, he can never leave.
because they're his family.
and who else has he got if not for them?