The second character is Mari, who is Momo's upperclassman by one year.
Her design took a long time to come about in a way that I was happy with. I think I had a specific idea in mind, but wasn't able to express it in the design for a while. The look above is a sort of blend of all my favorite elements from previous tries, dating back to around the same time I first started working on Momo.

I couldn't really find any older (or better) snapshots of the process, though I'm sure there must be some somewhere. That "first design" drawing was a recap sketch of my ideas up to that point, if I recall correctly.
Mari is tall and gentle, kind of like an older sister to Momo, at least at first glance. She's even more timid than her, but also has the composure to work through it (most of the time).