I say ENFJ for Jesse because he does not strike me as reserved in the least. Yes, he is not obnoxiously outgoing or intense, but that can be a false stereotype. Fe-doms often are good at being appropriate to the situation. He initiates conversation with a strange, attractive woman on a train - let's poll some INFx men and see how many would do that (outside of their imagination). He's got the trademark ENFJ charm. He convinces a bartender to give him a free bottle of wine, by basically appealing to some emotional feeling he knows this guy must be familiar with, as everyone is familiar with it in some way - Fe knows this.
His Fe is the most obvious function I see, but where Celine is random, he stops to shift his perspective, this is where you see his Ni kicking in. He's super accommodating, yet wants some definite plan - wasn't it he who suggested the 6 month idea? In Sunset, he wants to do something while they talk - go shopping, take the tourist boat ride.... This is an ENFJ kind of "spontaneity" - it's really a way of structuring their social environment. It's a way to control the backdrop, so to speak. Anyway, I could go on, but I'd need to watch it again to reference more specifics (I have seen it a gazillion times, but I'm not a big "detail" person when it comes to memory - much more a "gist" person).
Finally, he reminds me of many ENFJ men I've known, which is not much of an argument, but it's all too uncanny for me to dismiss personally

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I think her Fi is well-developed, but so is her Ne. I don't have any problem typing her as ENFP. I agree that is she is quite mature but still deliciously Fi + Ne, so that she might be called either NFP. An INFP typing would certainly have the caveat of her being exceptionally comfortable with the Jesse character. IDK who "Ariel" is - the Disney character? I don't relate to any Disney character myself....