Niki and her family were super nice and let me stay in their apartment, accepting a very spontaneous last-minute booking during off-season (right after Christmas).
Niki upgraded me to the bigger apartment (the Oceanographer) and made sure I was taken care of by her other family members, even if she wasn't on the island her self.
Thanks again Niki!
The apartment is super cute, tasteful decoration and had all the amenities as described. As for travellers looking into going to Serifos in December/January: It's ok, you can do it, however without proper heating it gets quite cold during the night. The Island is very windy during this time of the year. It's super beautiful though, if you are lucky with the weather as I was 2/3 days :)