I'm rewatching the first season.
The thoery of 9th as ISTP seems very probablle. Trying to understand reasons for everything, some issues with Fe and caring about feelings of people around him.
His journey is a bit like 12th as he learned to be more considerate of others.
But I'm not sure if 9th quick thinking is a result of his present-mindedness or an ability to connect ideas mentally.