I could cheat and just boringly say "herself"... but I'll give detail.
It's the haunting memory of Simca Develon.
Easily her most debilitating weakness. The sudden loss of Simca crippled Katrina and has left the entire window she saw the world through webbed with cracks in the glass. She questions everything now, everybody. Effects from the loss are an arm long list.
~ tumultuous relationships with her friends, often seeing the worst in everything they do and ignoring the best
~ severe decline in career performance resulting in her assignment away from the Syndicate zone
~ seclusion from most of the places she used to frequent, preferring only to float her liver at the I-RED Lounge
~ paranoia that borders on the superstitious now (she was already paranoid before)
~ frequent nightmares of losing Simca or others in various ways, often clearly at fault
~ "pod terrors", a form of waking nightmare or delusional panic attack while plugged into the capsule
~ decreased interest in diplomatic approaches, increased aggressive attitudes
She is known as the face of I-RED, and she's going insane. It's a miracle only few have noticed the signs and begun to question her merit. I say it's a miracle because that would only make things worse.
In my opinion, nobody and nothing is a greater threat and hinderance to Katrina than her inability to let go of Simca's memory.