Actually, I'm an INFJ and I've always perceived Leonardo as an ENFJ or an INFJ, my ultimate childhood crush. I loved him because he seemed so transparent to me: I felt like I could understand all his actions and I knew I'd have done the exact same thing.
He seemed to always find good in people, he was so incredibly stubborn about making friends with Karai in whom he
"sensed the good" (screaming Ni + Fe). He's a perfectionist too, tends to idealize the others and has a really fluctuating self esteem. He's the peacemaker in the group, the spiritual one who wanted to find and understand his true self in desolation. That's why he's never gotten along with Raph to whom the action is the real thing only. While watching all the seasons, I couldn't shake off the feeling of seeing my own reflection in the TV screen. Yet in the first season- he was broken, he had to spend a lot of the time alone to fully recharge after having been defeated by Shredder. It wasn't about wanting to heal his injuries: it was about healing his soul and trying to get over his perfectionism.
My father in law is an ISTJ and he's 100% unlike Leo- I have no idea where you guys see his S at all

He's a good person, but awkward in social situations, quite temperamental, EXTREMELY aware of his physical needs and, most importantly,
all but a leader.
Leo was such an interesting character to me- I especially loved him in the 2007 adaptation where he screamed NFJ. Value- oriented, responsible, extremely mature for his age, longing for peace and happiness of everyone in the group, insightful, unbelievably spiritual, driven but very hesitant to act unprepared. He doesn't use his Te at all- he tries to mobilize the team but is terrible at organizing them logically. He uses his Fe to get the team together. Neither does he use much of Si- he's completely unaware of his physical needs and he's focused on an infinite self- improvement for the better life of the group. What he cares about the most is becoming a better person in order to unite the brothers, become the eldest brother they need and live in a healthy family. He left for a very long time to a completely wild, dirty and unknown area just to be able to figure out who he is and who he should become. This is definitely not something which a Si- dom character does. In addition, in Central America, he was secretly helping the villagers resolve their problems with the thieves. A driven idealist, a grey emminence.
Note, that despite I'm a 100% INFJ (5w4), I am
not at all attracted to ENTPs. I find them too argumentative and I get exasparated very easily while talking to them- because I'm really bad at improvising coherent thoughts (I'm much better at writing them down) and they're loud and competent enough to always talk me out. I've always felt like they take pride in outsmarting people who are not necessarily less smart than the ENTPs, but just quieter or worse at speaking. The more you stutter, the better they feel about themselves. It irritates me beyond measure.
On the other hand, my two best friends are INTPs. There's been awesome chemistry, but as besties, not in a romantic way. I guess it could work romantically though.
My husband is an INFJ like me, but more of a loyalist (5w6). He's very similar to Leonardo, really. Just my type, I guess. I've always wanted to be understood on a very deep level- by someone who'd see things like I do. Intuition, self- development, insight, meaningful conversations about human nature. He's a true leader when the situation calls for it. He's much older than me and I was his first girlfriend- he was perceived as "the boring one"
Leonardo: INFJ / ENFJ (that's why he doesn't get along with Raph at all)
Raphael: ISTP
Donatello: INTP
Michelangelo: ESFP
My favorites:
1) Leonardo (a total crush, dedication, deep emotional understanding :blushed: )
2) Donatello (the best friend, a possible crush - to talk about intriguing stuff and inspire each other)
3) Raphael (a great friend)
4) Michelangelo (the little brother)