I finally have some time to do this properly, so let's do this! Brief disclaimer: I'm not very experienced with critiquing and everything I say is just a suggestion. I hope some of it is helpful!
I found that this sentence had a bit too much information for me to take in all at once. I'd suggest splitting it into two, but overall it's not a huge problem or anything. Additionally, I found it odd that the narrator already knows that their rival has a cut if it's being covered by the towel (I guess they could be using their see-through vision powers, but it's not mentioned)
Later on, he seems to be in quite a bit of pain, but I don't get the sense that he is from this introduction. I'm assuming he's trying to hide it, but it felt a little inconsistent to me. I'd suggest adding in a subtle sign that all is not well, maybe a brief wince as he cocks his head up? ("Hey," he said nonchalantly, cocking his head up and smiling at me, a slight wince flickering across his expression.)
Just another minor phrasing thing, but when I first read this sentence the "back" confused me. In my head I imagined it as the narrator sending their vision through the walls and then back to their own eyes, instead of them sending it back to the other room.
I was confused about who is speaking in this section. Originally, I thought it was the narrator, but on a second read I think it's the boy? To me, it sounds like the narrator is hinting that they don't believe his story. Also, if you want to, I think you could also convey his pain through speech, maybe a short gasp, or one of his sentences cuts off as the towel teleports off of his face? I don't think it's mandatory seeing as the nails part already communicates this information, but it wouldn't feel out of place.
This makes me smile every time I read it
I was confused for a bit, since I was imagining that he was sitting up the whole time. It could just be a me thing, though. If you wanted to change it, I would suggest maybe in the beginning where he plops down onto the sofa, a little bit could be added so it would read as something like "I saw him plop down on the nearest sofa, head against the cushions/armrest/that place on the sofa where you usually sit". Or maybe "I saw him plop down on the nearest sofa, not even bothering to sit up properly." Okay I think you get the idea I'm going to stop now.
More nitpicky stuff that probably doesn't matter, but when I read this aloud to myself, it felt more natural to laugh before the "Maybe I forgot." Honestly, I feel like a smirk/sly smile is more appropriate than a laugh, but that's just my personal preference. Or I could just be doing the laugh incorrectly.
Another personal preference thing that you can definitely ignore, but I feel like the water would leave a more lasting impression than the fall ("I scowled, remembering the icy plunge."?) (then again, we don't know what the fall was like, so my whole point could be invalid). Orrrrrrrrrrrr maybe a small element of humour because damp clothes are annoying and relatable which humanises the narrator more? "I scowled, remembering the soggy trip back to my apartment. He'd ruined my favourite [item of clothing], too." Or you could just leave it as is, it's really not a big thing.
I found this sentence a little tricky to read, since the "not" tripped me up. (This could also just be a me thing though). It could be changed to "Besides, when have you ever seen me kill someone outside of self-defense, or defense of the city?" (only if you want to, of course).
I like!
I only just thought about this now, but it would be interesting to see more about how the narrator feels about being woken up in the middle of the night, since I don't get the sense that they are up this late ordinarily. Are they annoyed or grumpy, still groggy from sleep? Does this change when they realise who's at the door? Or are they immediately alert, ready for danger? This could be in the opening, or interspersed throughout the text.
I really like this, but sweet taffy brings connotations of, a nice, relaxed silence. I don't know if that's what you were going for, and it still works if that is the case, just wanted to bring it to your attention in case you wanted it to be a bit more tense or awkward.
I really like this too -- it raises questions. Did he plan this all from the beginning with the intent to trap her? Or did he see an opportunity and take it? It exposes a more devious side of him that we may not have gotten before, and it's a somewhat jarring reminder that they're still enemies. (I don't know why I typed that all out, seeing as you probably already know that)
I don't have any problems with this, it's a solid closing line, but just as a suggestion, something about her getting him back (in a lightheartedish way, not like going to kill him or anything) could be fun. I'm thinking something along the lines of "Well, he'd better watch his back." But that's just a suggestion (and another me thing).
Also, just a general comment, but I think there could be some more variation of sentence lengths, as I think most of them are quite long. The writing still flows nicely despite this though!
All in all, it's a really fun and engaging story, and I enjoyed both characters and their relationship with one another. I think you explored it really well, and I'm excited to read to prequel