User blog comment:Jopjopjop/Volume 19 Spoilers/@comment-16281343-20141119210121/@comment-1359322-20141119224215

How is Tsubaki idiotic for viewing Issei as a rival? Even if Issei doesn't swing that way, it can't deny the fact that Kiba is closer to Issei in fact Kiba tends to lean to Issei for moral support. Issei can lean on Rias for moral support when he needs, but Kiba leans on Issei. It's only natural for Tsubaki to view Issei as a rival, she needs to take over that position from Issei before she can make any progress.

Plus Issei is way closer to Kiba than she is, hell the best way to describe Tsubaki and Kiba's relationship as of now is merely comradeship or allies at best, not even friendship like Issei and Kiba are at. Relationship mostly starts from friendship and Tsubaki isn't even at the starting point.