Yeah. If you look at a crossover with a game about a funny fat guy and immediately screech about "NTR! NTR! NTR!", you should probably read less Hentai, get a bit to eat, and get some fresh air.
Also, knowing the track record of Nikke when it comes to crossovers, there's likely nothing going to happen in that regard with Dave anyhow, be it romantic or sexual. Crossover event stories generally tend to be rather flat, with the closest thing to an interesting story being the ones that actually don't involve the MC and his Squad at all. Such as in the Nier Automata Event, where A2 bonding with a mass-produced Nikke over taking care of a little bird, or the robot Pascal making friends with a dying Nikke.
All that's likely gonna happen is an inciting incident, some introductions between Dave and the Nikke cast, some slapstick about diving and/or beach shenanigans (Summer Events tend to be more light-hearted), some minor tension due to the inciting incident, the inciting incident is resolved, and then some beers, snacks, and farewells. Not much more than that.