I've seen a lot of people call Yuujin bland but I've always had the impression that he //is// bland exactly because of his purpose, his reason for existing, and the plot. I would be very interested in what you have to say!
I think bland is a bit much to call it, personally. (Spoilery talk under the cut.)
I do think they had to keep his motives and feelings a bit ambiguous for the sake of the endgame plot twist so that you can believe for a second that everything really was all a lie, but once it's established for good that it isn't per se, there's quite a bit you can unpack retroactively. For instance, what he said in the past about why he values and looks up to Haru so much; it's true that the line is blurred in terms of how much his personality was a construct to cater to Haru, but that didn't make it any less real.
The one consistent thread we see with all of Yuujin's personal thoughts is that he's actually pretty insecure and lacking in self-confidence, mainly due to his past experiences of having been too forceful regarding pushing his way on others and turning them off. This is why he looks up to Haru for being "kind"; Haru is much more conciliatory and able to bring people together without being aggressive and in-your-face. Yuujin (who, if you'll remember, is supposed to reflect the more stereotypical brash Digimon protagonist from prior series) had rubbed people the wrong way in the flashbacks presented in episodes 18 and 29, and in those cases Haru had been the only one to cut him slack and help him make up with the others.
I think it's easy to think that Yuujin's life revolves exclusively around Haru, especially since he was literally made for that purpose, but the thing is that he says explicitly that "I just wanted to have fun with everyone." If Haru had legitimately been his only world, he wouldn't have minded being ostracized from the others because Haru would still have been by him, but Haru helped him make friends with others. This is also especially in consideration of the fact that, when you think about it, "Yuujin" as a separate entity from YJ-14 probably wasn't even supposed to exist in the first place -- it's just that at some point the "simulation" Leviathan created to cater to Haru started to become so intimate of a simulation that it became indistinguishable from the real thing, capable of acting on its own will.
The other thing is that everyone's Buddy Appmon can be considered to be reflections of themselves in some way; it may not be literally psychological the same way it is in Adventure/02, but as Gatchmon himself says in episode 49, the kids and their Buddy Appmon were brought together because they have common goals. Haru is curious and open-minded, and interested in knowing more, hence he's paired with Gatchmon, the curious search engine Appmon who wants to know about everything; Eri and Dokamon both have strong and aggressive personalities, but all of it is for the sake of being endearingly loyal and to protect what they care about; Astra and Musimon both live by their feelings and trying to figure out what it is they want; Rei and Hackmon both have cold exteriors and are willing to play dirty, but do what they do out of loyalty and passion. So when you look at Yuujin and Offmon, they're not actually that different past the surface; Offmon is a bit sheltered from knowing the full extent of the world and very insecure in himself, but also capable of doing great things, either for good or for evil -- all much like Yuujin, who may be a byproduct of a very narrow purpose but still works from a well-meaning desire to protect others. Also, one thing that might strike you if you rewatch the final episode: the first thing Yuujin does upon being released from Leviathan's control is break down and start crying, becoming pessimistic about his ability to make up for his actions and apologizing profusely all the way down, which may bring to mind the habits of a certain other someone...
