"You're no longer playing with me now but instead with them."
Maybe I shouldn't say anything since I'm not a native English speaker, but since this is but a manga forum, I'll say it nonetheless: Something about that line looks artificial in a casual conversation like in the chapter. I know it's to mask the personal pronoun spoiling the twist, which wouldn't be a problem in Japanese, but maybe it could have been handled in a different manner. Because in this situation Kinoshita clearly sees Kento is only with one person who's right there, so there's no "them".
Maybe it could be something like: "... but instead with others." You could ask what the plural is doing there, but it doesn't look unnatural. If Kento is spending time with one person, despite telling he's too busy, then why not with more people? That's the kind of accusation the offended party often makes. It's a kind of generalisation to make the matter look a bit worse even if only a single instance was wittnessed, an assumption that the offender must have done it more but was only caught by accident this one time.