I think it's Froppy
-All Might says "truly intelligent villains hide in the shadows" here at the 10:10 mark https://youtu.be/IBWfT6_huo8
Froppy can be later seen using the shadows which could be possible foreshadowing 5:23 https://youtu.be/X8I6GHnwc1k
-Froppy says your quirk is a lot like All Might's which seems suspect to me like she knows a lot more than she's letting on 8:20 https://youtu.be/X8I6GHnwc1k
-Not too long after that on the boat Deku has a somewhat similar almost telling moment when he says the villains must have underestimated them not knowing their quirks when he, Mineta, and Froppy all get warped onto the boat but the league could have done that to give Froppy the advantage in water as well as to help maintain her cover.
-Froppy is one of the few students rooms we don't see and her behavior is somewhat suspect when she pops her head out around 1:18 https://youtu.be/vR19Y7gnsH0
I also kinda read into this as her being torn between being a villain and hero, like deep down maybe she's struggling with being a double agent/traitor and at that time she just wanted to play the room game and fit in to have fun. Obviously she couldn't though like in the event she was hiding special secret equipment to help stay in touch with the league, AfO, or whomever.
-She:s very talented, highly capable of many things, intelligent, hasn't lost a fight yet, somewhat OP some might argue especially during the class battles, and she just seems somewhat shady and suspicious to me.
-She's one of the select few that got to do an internship during the Yakuza arc and was directly involved with the Eri case. She may have tipped off the league as to what was going on therefore giving them the upper hand against Overhaul and the rest of the Yakuza, the league did bounce and seemed to know when to get out just in time before shit hit the fan. Also someone tipped them off in the beginning.
-Has camouflage so could easily sneak in and out of places undetected.