I think I mention in Sei's bio, but when I draft up Lein's I'll mention it again: the two have an odd relationship that fluctuates between rivalry and couple. Most of it is they're both bad at managing distance in relationships. On the one hand, Sei is bad at recognizing social cues and her playful demeanor and ditziness sends mixed signals. On the other, Lein is a bit too girl-crazy and Sei's the first person she's wanted to have as a friend since... well, since Jun and Rie. The Yeni doesn't want to ruin it, so she's often having to fight herself over what to say or do in any given scenario.

At minimum, the two of them are a cute yuri tease, but at most it does lead to a relationship you might have seen a few times in daily life. The awkward 'Are they a couple or just close friends?' type where it's hard to tell.