Def an attachment type, which shows itself romantically, for example, when he can’t quite break up with the insane water-twin. Also reveals itself in the final season with Kuvira, with his loyalty to her.
Bolin is sweet, devoted, an all around good guy. Could be a 9, but he is the comic relief of the show, and talks fast and zigzaggingly in the style of 6s. He doesn’t have the withdrawn energy of 9s and wants to do the right thing, like a compliant type. His character arc is concerned with individuating, not being the younger brother that needs to be taken care of so much. He often self-sabotages his move towards independence by doing dumb things he needs to get rescued from (like working with the street gang that leads him to Aman) and getting involved with domineering personalities, like the water twin and Kuvira. He second guesses himself often but is courageous in the style of 6s, as we see on numerous occasions, for example when he eventually rebels against Kuvira.