In the first episode, Sakakibara saw Misaki in the basement of the hospital. I've seen the first 9 episodes and I still didn't get why she was there and what she was doing.
Mei was "visiting" her twin sister Misaki Fujioka
She dies in Episode 1 from the 9th grade class 3-3 curse, being "the death of April", the first one who died because of the curse. Mei was delivering a present at the Yomiyama Hospital's morgue, a doll, Mei referred to her as "her other half" during her first meeting with Kouichi and later as her twin sister.
Source: Misaki Fujioka - Plot - Another
In the anime/ova Fujioka died from leukemia (flared up in the OVA) but is still counted as being one of the deaths caused by the calamaty. in the novel she had a kindey transplant and Mei was going to gift her the dool but died before she could
In the original novel, the doll that Mei brings to the morgue was a gift to Misaki after her kidney transplant. The kidney Misaki received was not Mei's (due to minors not being allowed to donate). Misaki told Mei which doll she wanted but died before Mei could bring it to the hospital.
Fujioka is also mentioned near to the end of the series when Takako thinks Mei is the "Extra" because she remembers going to school with a Misaki but confuses Fujioka with Mei and remembers that Fujioka (thinking she's Mei) didn't have different coloured eyes (one of Mei's eyes is a prostetic doll's eye)