Aikawa-senpai 's name is

Aikawa Ruri.

A second year student attending Tokyo Jujutsu High ' and the only second year student in the school, as it turns out.

That 's quite unfortunate. Being the only student in her class, that is; a fate that Shiki herself had only narrowly escaped thanks to Hoshi-san. It does help to explain why Aikawa-senpai had volunteered to watch a first year underclassman run up and down the mountain stairway the other day.

'Loneliness. ' Or something akin to that, Shiki surmises.

' Or it could also be that Aikawa-senpai simply doesn 't mind doing small favors for Takagi-sensei here and there. It 's not particularly difficult to notice the easy way they converse with each other, the way that Aikawa-senpai is comfortable in Takagi-sensei 's presence and smiles as she speaks to him. From what Shiki can see, it 's clear that Aikawa-senpai trusts and likes Takagi-sensei.