Thinking about freak4freak AiDeku. Izuku wouldn’t treat his significant other much different from how he treats everyone else, he would just do everything with more intensity. Meanwhile, Shouta would 100% be different with his significant other—it’s like a 180. However, both of their methods of loving would not be considered appropriate to the bulk of society (aka they would 100% be yanderes).
Izuku would be oblivious about his tendencies. Not only does he have multiple notebooks of information on Eraserhead, but he also has them on Shouta’s civilian identity. Izuku doesn’t have a concept on what information he should or shouldn’t be privy to, so he has records from Shouta’s birth all the way to present. He has notes on things that Shouta isn’t even consciously aware of himself yet. He doesn’t take any candid photos. God no! People might have called him a creep all his life, but it’s not like he actually is one. Instead, he has multiple sketchbooks filled with in depth drawings of Shouta. This is outside of what he already has sketched within his Eraserhead volumes.
Izuku knows what he’s doing is a bit much even for him, but he wouldn’t be such a great analyst if he didn’t go the full nine yards. Eraserhead is just such an amazing hero! Who wouldn’t find out everything they could about him? However, Izuku acknowledges that his work can be off-putting even when it's required, so he doesn’t advertise the extra Eraserhead volumes and definitely not the ones on Shouta’s civilian identity. To the random stranger that runs into Izuku once, his actions make him seem insane. To anybody who remotely knows him, this behavior is no different from how the analyst normally operates.
Shouta has known all his life that his tendencies aren't normal for the bulk of society, much to his displeasure. It never did make much sense to him why people were so inefficient. If you care about something and don’t want it broken then you keep it close. You do what you have to in order to keep your things close by and in tip top condition. If that requires some on-site research and action, well Shouta is just taking the most efficient and logical path. All the information he obtains and everything he does is for the betterment of what’s his. And when he decides that something is his, he never lets go. Fortunately—or unfortunately depending on how you look at it, Shouta couldn’t care less about the majority of the world beyond surface level.
Shouta weaves his way into Izuku’s life bit by bit. A coincidental encounter at the agency Izuku works at—Izuku is the analyst on a case Shouta’s working on. Bringing him little gifts that Izuku can’t refuse because it’s all necessities—food and the like. Showing subtle possession over Izuku to others through the guise of little acts of kindness. While on patrol, Shouta runs into Izuku and offers to walk him back home since his commute just so happens to coincide with Shouta’s patrol route. It’s no trouble at all and refusing would be illogical, so Izuku doesn’t. Shouta threads himself into every facet of Izuku’s life to the point that if he were for some reason no longer a part of it, it would fall apart. To the random stranger that runs into Shouta once, his actions are just those of a kind hero. To anybody who remotely knows him, this behavior is the first sign of insanity.