The beginning of Kannazuki no Miko is something a little uncommon; all three parts of the love triangle already know each other, but haven’t expressed feelings beyond friendship. The events of the first episode change this for both Chikane and Souma, though Souma effectively has a head start because he’s much more direct.
At first, Himeko and Chikane’s relationship seems barely changed from the secret friendship they already had, and Chikane tries to support what Himeko wants even if it’s being with someone else, but she gradually grows more conflicted about it.
Even while seeing their relationship as friendship rather than romance, Himeko views Chikane in an overly idealized way. But Chikane herself would be the first to say she isn’t perfect, and even take it too far into the opposite direction; it’s never explicitly spelled out, but Chikane suffers from some degree of internalized homophobia. Chikane also simply assumes (falsely) that Himeko, who she idealizes, could never have such “dirty” feelings in return.
In fact, it’s interesting to note that Himeko saying she’s realized Chikane is a mix of good and bad is associated with her romantic feelings, in contrast to many other series where idealization is associated with romance.
Alright, I’m going to be talking about Chikane’s actions in the later part of the series next; major spoilers obviously, though to be honest, this main one is the “Rosebud” of yuri.