ASUKA
Becoming close friends with both Terashima Yuhi and Ibuki Sumire meant she would notice signs about their relationship that others wouldn't. One of those things being, 'When they are apart from one another, they end up doing reckless things.' Terashima Yuhi collapsing from a fever, he probably didn't even know he had wasn't surprising.
Asuka's gaze landed on Yuhi who was breaking out into a fever. Sumire told her about Aki's theory about her heart being connected, tethered to his and Asuka couldn't deny it. Because there were times in the past when Sumire was unwell, and Aki wasn't well either. It happened far too many times for it to be a coincidence.
But, it's not just Sumire. The other day the captain pulled her aside for a talk, and spoke about Yuhi's decreasing power levels.