As far as I can tell, at this point three girls have feelings for No 6
- Alice, who he called her partner, which made her feel nice
- Snow, after he saved her kingdom even though he was a spy
- Grimm, because he stayed with her when she was being revived.
I think it could be the harem route, but it's still too early.
alice,grimm and the demon lord girl are top contenders for now.Snow is just comedy material at this point;on the orher hand rose feels more like a partner than a love interest.
ranking wise;
it would be Grimm,demon lord girl (she is acting a bit dere around 6;which is her personality's stark opposite),and then alice.she seems to have some positive feelings for 6, though in the latest volume; there's almost no trace of it.
So far I've read the first 5 volumes, and based on just that I feel this will have sort of an open ended harem. Almost all of them have a screw or two loose.
IMO these are the 3 biggest contenders till now:
1) Astaroth (one of the supreme leaders, clearly shown to have feelings for MC, and it's implied MC likes her too. Initially the MC just wanted to finish the job quickly and go back to her side.
2) Grimm is silly, but probably the boldest about her feelings, she clings to the MC
and has already forced him into an engagement of sorts.
3) Viper (Demon Lord's daughter) - She seems to be the most stable and kind out of all, and MC is shown to care quite a bit for her
to the point where he was willing to risk his entire mission and go against everyone, including his partner Alice, to save her from execution.
It's probable that she likes him, given how she's most comfortable around him (out of all the non-demons)
Initially I thought Snow would be the one of the 2 key love interests along with Astaroth, but she has devolved into a hopelessly greedy character, though she is known to get her priorities right in times of severe crisis.
P.S. I don't see Alice as a love interest in this series, there's nothing to show that they have any romantic feelings for each other. Her AI personality is based on the MC which is why they get along well like partners in crime.
But if the author follows KonoSuba's route, Alice could indeed end up as his main love interest despite there being a plethora of attractive/busty girls, which would be pretty disappointing
I want Grimm or the demon lord aka Vi Vi to end up though it's like isekai so it might be a harem route though I don't see Alice and Snow ending up with No 6