CoNeTrIC:
Hard to say. Misogyny is normalized in the world of BL, I'm sad to say. This character is more nuanced and sympathetic than the typical female BL character and, still, people get off on hating her. Lots of wasted emotion.
Imagine a supporting character who is interested in the main lead.
The character forces people around the main lead to share information on the main lead, but do not answer even the basic question they face. They threaten the Main Lead's love interest, overexaggerate their own relationship with Main Lead, and diminish the Main lead's interest/attention/affection for their love interest. They expose private information about the main lead and/or the Main lead's Love interest in a public forum.
It doesn't matter if this supporting character is male or female, this person deserves contempt.
There are Female Supporting Characters in BL, like Peal from Gameboys, Dev from Boy's Lockdown, but there are also characters like Sandee Mendoza from My Extraordinary. At the same time, there are also Male Support Characters in BL, like Shin Woo from this show, but there are also characters like Non from Dark Blue Kiss.